Cluster supported catalyst and production method therefor

ABSTRACT

A cluster-supporting catalyst including porous carrier particles having acid sites, and catalyst metal clusters supported within the pores of the porous carrier particles. In the cluster-supporting catalyst including porous carrier particles having acid sites, and catalyst metal clusters supported within the pores of the porous carrier particles, the catalyst metal may be rhodium, the catalyst metal may be palladium, the catalyst metal may be platinum, or the catalyst metal may be copper.

This is a Divisional Application of application Ser. No. 16/066,928filed Jun. 28, 2018, which is a National Phase of InternationalApplication No. PCT/JP2016/088792 filed Dec. 26, 2016, which claims thebenefit of Japanese Application No. 2015-257320 filed Dec. 28, 2015. Thedisclosures of the prior applications are hereby incorporated byreference herein in their entireties.

FIELD

The present invention relates to a cluster-supporting catalyst and aproduction method thereof. More specifically, the present inventionrelates to a cluster-supporting catalyst for exhaust gas purification,liquid-phase chemical synthesis reaction, gas-phase chemical synthesisreaction, fuel cell reaction, etc., and a production method thereof.

BACKGROUND

A supported catalyst obtained by supporting a catalyst metal on acarrier is used in many fields and is used as a catalyst for exhaust gaspurification, liquid-phase chemical synthesis reaction, gas-phasechemical synthesis reaction, fuel cell reaction, etc.

As to such a supported catalyst, it is known that the size of thecatalyst metal particles supported on a carrier is important. In thisconnection, for example, Patent Document 1 has proposed a supportedcatalyst in which catalyst metal particles having a size of 1 to 10 nmare supported on a carrier of alumina, silica, titania, zirconia or acombination thereof. In addition, Patent Document 2 has proposed acatalyst for nitrogen oxide, in which copper ion is supported on zeoliteby ion exchange. Furthermore, Patent Document 3 has proposed a techniquewhere palladium supported on zeolite by ion exchange is dried and thenclustered by a reduction treatment to prepare a cluster-supportingcatalyst, and the cluster-supporting catalyst is used for a couplingreaction, etc.

RELATED ART Patent Document

[Patent Document 1] JP2006-212464

[Patent Document 2] JP2012-148272

[Patent Document 3] JP2010-69415

SUMMARY Problems to be Solved by the Invention

Although various supported catalysts have been proposed as describedabove, a supported catalyst having further improved catalytic activityis demanded. In this connection, the cluster-supporting catalystdescribed in Patent Document 3 can exhibit reaction properties differentfrom conventional catalysts, but there is room for improvement in regardto the heat resistance. Among others, in the field of exhaust gaspurification, a supported catalyst having an improved low-temperatureactivity for nitrogen oxide (NOx) reduction and/or carbon monoxide (CO)oxidation after thermal endurance is demanded.

Accordingly, the present invention provides a cluster-supportingcatalyst having improved heat resistance, and a production methodthereof.

In addition, the present invention provides a method for evaluating thesize of catalyst metal particles in a supported catalyst.

Means to Solve the Problems

The present inventors have found that a specific cluster-supportingcatalyst has improved heat resistance, and arrived at the presentinvention described below.

<Embodiment 1> A method for producing a cluster-supporting catalyst,

wherein the cluster-supporting catalyst comprises porous carrierparticles having acid sites, and catalyst metal clusters supportedwithin the pores of the porous carrier particles; and

wherein the method comprises the followings steps:

providing a dispersion liquid containing a dispersion medium and theporous carrier particles dispersed in the dispersion medium, and

forming, in the dispersion liquid, catalyst metal clusters having apositive charge, and supporting the catalyst metal clusters on the acidsites within the pores of the porous carrier particles through anelectrostatic interaction.

<Embodiment 2> The method according to embodiment 1, wherein thedispersion liquid is provided by pulverizing the porous carrierparticles, and dispersing the pulverized porous carrier particles in thedispersion medium.

<Embodiment 3> The method according to embodiment 1 or 2, wherein theclusters are formed in the dispersion liquid by any of the followingmethods:

a method of laser ablation in liquid,

a method of microwave ablation in liquid,

a method of plasma ablation in liquid, and

a positive-negative inversion method.

<Embodiment 4> The method according to embodiment 1 or 2, wherein theclusters are formed in the dispersion liquid by a method of reduction inliquid.

<Embodiment 5> The method according to embodiment 4, wherein thedispersion liquid is irradiated with plasma and/or microwave to promotereduction of an ion of the catalyst metal by the reducing agent.

<Embodiment 6> The method according to any one of embodiments 1 to 4,wherein the dispersion medium of the dispersion liquid is an organicsolvent.

<Embodiment 7> A method for evaluating the size of catalyst metalparticles supported in a supported catalyst,

wherein the supported catalyst comprises porous carrier particles andcatalyst metal particles supported on the porous carrier particles, and

wherein the catalyst metal particle size evaluation method comprises thefollowing steps:

providing a dispersion liquid having dispersed therein the supportedcatalyst, and

evaluating the size of the catalyst metal particles in the supportedcatalyst based on the presence or absence of fluorescence emitted fromthe supported catalyst by irradiating the dispersion liquid withexcitation light.

<Embodiment 8> The method according to embodiment 7, further comprisingreduction-treating the catalyst metal particles before evaluating thesize of the catalyst metal particle.

<Embodiment 9> A cluster-supporting catalyst comprising porous carrierparticles having acid sites, and catalyst metal clusters supportedwithin the pores of the porous carrier particles, wherein

the catalyst metal clusters are obtained by supporting catalyst metalclusters having a positive charge, which is formed in a dispersionliquid containing a dispersion medium and the porous carrier particlesdispersed in the dispersion medium, on the acid sites within the poresof the porous carrier particles through an electrostatic interaction.

<Embodiment 10> A cluster-supporting catalyst comprising porous carrierparticles having acid sites, and catalyst metal clusters supportedwithin the pores of the porous carrier particles, and satisfying any ofthe following (a) to (d):

(a) the catalyst metal is rhodium and satisfies at least one of thefollowing (a1) to (a3):

(a1) when the cluster-supporting catalyst is subjected to a firstthermal endurance treatment for rhodium, then to an oxygen adsorptionpretreatment and further to a test by hydrogen temperature-programmedreduction method, the peak of reaction between hydrogen supplied andoxygen adsorbed to the cluster-supporting catalyst is present in thetemperature range of 150° C. or less,

wherein the first thermal endurance treatment for rhodium is a treatmentof subjecting the cluster-supporting catalyst to heating for 2 hours inan atmosphere at 800° C. containing 20 vol % of oxygen and the balancehelium and then to heating for 1 hour in an atmosphere at 800° C.containing 0.5 vol % of hydrogen and the balance helium,

the oxygen adsorption pretreatment is a treatment of adsorbing oxygen tothe cluster-supporting catalyst at 30° C. for 1 hour in an oxygenatmosphere and removing excess oxygen at 500° C. for 1 hour in a heliumatmosphere, and

the test by hydrogen temperature-programmed reduction method is a testof flowing a reducing gas containing 0.5 vol % of hydrogen and thebalance helium at a spatial velocity of 10,000 h⁻¹ over thecluster-supporting catalyst while raising the temperature at a rate of10° C./min from 20° C.;

(a2) when the cluster-supporting catalyst is subjected to a secondthermal endurance treatment for rhodium and then to a nitric oxidereduction test, (i) the reaction temperature at the time of half of thenitric oxide supplied being reduced into nitrogen satisfies at leasteither one of 300° C. or less in the temperature rising process and 270°C. or less in the temperature dropping process, and/or (ii) the numberof molecules of the nitric oxide molecule when nitric oxide can bereduced by one rhodium atom at a temperature of 250° C. in thetemperature dropping process is 0.005 molecules/sec or more,

wherein the second thermal endurance treatment for rhodium is atreatment of heating the cluster-supporting catalyst for 1 hour in anatmosphere at 800° C. containing 8 vol % of oxygen, 0.3 vol % of carbonmonoxide and the balance helium, and

the nitric oxide reduction test is a test of flowing a model gascontaining 0.1 vol % of ¹⁵NO, 0.65 vol % of CO and the balance helium ata spatial velocity of 10,000 h⁻¹, performing a temperature risingprocess of raising the temperature at a rate of 10° C./min to 800° C.from room temperature, and then performing a temperature droppingprocess of lowering the temperature to room temperature; and

(a3) when the cluster-supporting catalyst is subjected to a cleaningtreatment and then to an adsorbed carbon monoxide oxidation test, thepeak of reaction between carbon monoxide adsorbed to thecluster-supporting catalyst and oxygen in the atmosphere is present inthe temperature range of 200° C. or less,

wherein the cleaning treatment consists of the following steps (i) to(iv):

(i) putting the catalyst at a concentration of 4 mass % in an aqueous 1M sodium chloride solution, followed by stirring at 80° C. for 10 days,

(ii) after (i) above, rinsing the catalyst with ion-exchanged water,

(iii) after (ii) above, putting the catalyst at a concentration of 4mass % in an aqueous solution containing 6 mass % of polyoxyethylenesorbitan monolaurate, 0.25 M trisodium ethylenediaminetetraacetate, and0.01 M sodium borohydride, followed by stirring at 80° C. for 10 days,and

(iv) after (iii) above, rinsing the catalyst with ion-exchanged water,and

the adsorbed carbon monoxide oxidation test is a test of adsorbingcarbon monoxide to the cluster-supporting catalyst by holding thecluster-supporting catalyst at 800° C. for 1 hour in an atmospherecontaining 500 ppm by volume of carbon monoxide and the balance helium,and thereafter oxidizing the carbon monoxide adsorbed to thecluster-supporting catalyst into carbon dioxide with oxygen by heatingthe cluster-supporting catalyst having adsorbed thereto carbon monoxideat a rate of 10° C./min to 800° C. in an atmosphere containing 10 vol %of oxygen and the balance helium;

(b) the catalyst metal is palladium and

when the cluster-supporting catalyst is subjected to a thermal endurancetreatment for palladium and then to a carbon monoxide purification testfor palladium, the number of molecules of the carbon monoxide moleculecapable of being oxidized to carbon dioxide molecule by one palladiumatom is 0.004 molecules/sec or more,

wherein the thermal endurance treatment for palladium is a treatment ofheating the cluster-supporting catalyst for 10 hours in an atmosphere at800° C. containing 20 vol % of oxygen and the balance helium, and

the carbon monoxide purification test for palladium is a test of flowinga model gas containing 0.3 vol % of carbon monoxide, 8.0 vol % of oxygenand the balance helium at a spatial velocity of 10,000 h⁻¹ over thecluster-carried catalyst, performing a temperature rising process ofraising the temperature at a rate of 10° C./min to 800° C. from roomtemperature, then performing a temperature dropping process of loweringthe temperature to room temperature, and measuring the catalyticactivity at a temperature of 100° C. in the temperature droppingprocess;

(c) the catalyst metal is platinum and satisfies at least one of thefollowing (c1) and (c2):

(c1) when the cluster-supporting catalyst is subjected to a thermalendurance treatment for platinum and then to a carbon monoxidepurification test for platinum, the number of molecules of the carbonmonoxide molecule capable of being oxidized to carbon dioxide moleculeby one platinum atom is 0.00015 molecules/sec or more,

wherein the thermal endurance treatment for platinum is a treatment ofheating the cluster-supporting catalyst for 10 hours in an atmosphere at800° C. containing 20 vol % of oxygen and the balance helium, and

the carbon monoxide purification test for platinum is a test of flowinga model gas containing 0.3 vol % of carbon monoxide, 8.0 vol % of oxygenand the balance helium at a spatial velocity of 10,000 h⁻¹ over thecluster-carried catalyst, performing a temperature rising process ofraising the temperature at a rate of 10° C./min to 800° C. from roomtemperature, then performing a temperature dropping process of loweringthe temperature to room temperature, and measuring the catalyticactivity at a temperature of 60° C. in the temperature dropping process,and

(c2) when the cluster-supporting catalyst is subjected to a cleaningtreatment and then to an adsorbed carbon monoxide oxidation test, thepeak of reaction between carbon monoxide adsorbed to thecluster-supporting catalyst and oxygen in the atmosphere is present inthe temperature range of 200° C. or less,

wherein the cleaning treatment consists of the following steps (i) to(iv):

(i) putting the catalyst at a concentration of 4 mass % in an aqueous 1M sodium chloride solution, followed by stirring at 80° C. for 10 days,

(ii) after (i) above, rinsing the catalyst with ion-exchanged water,

(iii) after (ii) above, putting the catalyst at a concentration of 4mass % in an aqueous solution containing 6 mass % of polyoxyethylenesorbitan monolaurate, 0.25 M trisodium ethylenediaminetetraacetate, and0.01 M sodium borohydride, followed by stirring at 80° C. for 10 days,and

(iv) after (iii) above, rinsing the catalyst with ion-exchanged water,and

the adsorbed carbon monoxide oxidation test is a test of adsorbingcarbon monoxide to the cluster-supporting catalyst by holding thecluster-supporting catalyst at 800° C. for 1 hour in an atmospherecontaining 500 ppm by volume of carbon monoxide and the balance helium,and thereafter oxidizing the carbon monoxide adsorbed to thecluster-supporting catalyst into carbon dioxide with oxygen by heatingthe cluster-supporting catalyst having adsorbed thereto carbon monoxideat a rate of 10° C./min to 800° C. in an atmosphere containing 10 vol %of oxygen and the balance helium; and

(d) the catalyst metal is copper and

when the cluster-supporting catalyst is subjected to a nitric oxidetemperature-programmed desorption test, the maximum peak in the range of200 to 400° C. is present in the range of 200 to 300° C.,

wherein the nitric oxide temperature-programmed desorption test is atest of adsorbing nitric oxide to the supported catalyst through heatingat 800° C. for 1 hour in an atmosphere containing 10 vol % of oxygen andthe balance helium, heating at 800° C. for 30 minutes in an atmospherecontaining 100 vol % of helium, lowering of the ambient temperature to25° C., holding for 1 hour in an atmosphere containing 500 ppm by volumeof nitric oxide and the balance helium, and holding for 1 hour in anatmosphere containing 100 vol % of helium, and thereafter heating thesupported catalyst having adsorbed thereto nitric oxide at a temperaturerise rate of 10° C./min to 800° C. in an atmosphere containing 100 vol %of helium.

<Embodiment 11> The cluster-supporting catalyst according to embodiment10, which satisfies (a) above.

<Embodiment 12> The cluster-supporting catalyst according to embodiment10, which satisfies (b) above.

<Embodiment 13> The cluster-supporting catalyst according to embodiment10, which satisfies (c) above.

<Embodiment 14> The cluster-supporting catalyst according to embodiment10, which satisfies (d) above.

<Embodiment 15> The cluster-supporting catalyst according to any one ofembodiments 9 to 14, wherein the catalyst metal supporting rate in poresas defined by the following formula is 62.5 mol % or more:

Catalyst metal supporting rate in pores (mol %)=B/A

A: the number of atoms (mol/g) of all the catalyst metal supported onthe porous carrier particles,

B: the number of atoms (mol/g) of the catalyst metal, determined byeither the following evaluation standard (B1) or (B2):

(B1) the number of atoms (mol/g) of the catalyst metal obtained bysubtracting the catalyst metal present as one atomic ion and thecatalyst metal supported on the outer surface of the porous carrierparticles, from the number of atoms of all the catalyst metal supportedon the porous carrier particles, or

(B2) the number of atoms (mol/g) of the catalyst metal supported on theporous carrier particles after the following treatments (i) to (iv):

(i) putting the cluster-supporting catalyst at a concentration of 4 mass% in an aqueous 1 M sodium chloride solution, followed by stirring at80° C. for 10 days,

(ii) after (i) above, rinsing the cluster-supporting catalyst withion-exchanged water,

(iii) after (ii) above, putting the cluster-supporting catalyst at aconcentration of 4 mass % in an aqueous solution containing 6 mass % ofpolyoxyethylene sorbitan monolaurate, 0.25 M trisodiumethylenediaminetetraacetate, and 0.01 M sodium borohydride, followed bystirring at 80° C. for 10 days, and

(iv) after (iii) above, rinsing the catalyst with ion-exchanged water.

<Embodiment 16> The cluster-supporting catalyst according to any one ofembodiments 9 to 15, wherein the porous carrier particles is a particleof a microporous material.

<Embodiment 17> The cluster-supporting catalyst according to any one ofembodiments 9 to 16, wherein the porous carrier particles is a zeoliteparticle.

<Embodiment 18> The catalyst according to any one of embodiments 9 to17, which is an exhaust gas purification catalyst.

<Embodiment 19> The catalyst according to any one of embodiments 9 to18, which is a catalyst for liquid-phase synthesis reaction, gas-phasesynthesis reaction or fuel cell reaction.

<Embodiment 20> A catalyst device comprising the catalyst according toany one of embodiments 9 to 19 and a substrate supporting the catalyst.

Effects of the Invention

According to the cluster-supporting catalyst of the present invention,an improved catalytic activity can be provided. Furthermore, accordingto the method of the present invention for evaluating the size ofcatalyst metal particles, the size of catalyst metal particles can beevaluated in a supported catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are diagrams illustrating one embodiment of themethod for producing the cluster-supporting catalyst of the presentinvention.

FIG. 2A and FIG. 2B are graphs illustrating the fluorescence spectrawith respect to the supported gold-cluster catalysts of Examples 1 and 2and Comparative Example 1.

FIG. 3 depicts graphs illustrating the fluorescence spectra beforeoxidation treatment and reduction treatment with respect to thesupported copper-cluster catalyst of Example 3.

FIG. 4 is a graph illustrating the temperature change for the evaluationof catalytic activity.

FIG. 5A and FIG. 5B depict graphs illustrating the carbon monoxidepurification performance with respect to the rhodium cluster-supportingcatalysts of Example 4 and Comparative Example 2; FIG. 5A is the resultsof the temperature rising process and FIG. 5B is the results of thetemperature dropping process.

FIG. 6A and FIG. 6B depict graphs illustrating the carbon monoxidepurification performance with respect to the rhodium cluster-supportingcatalysts of Example 5 and Comparative Example 3; FIG. 6A is the resultsof the temperature rising process and FIG. 6B is the results of thetemperature dropping process.

FIG. 7A and FIG. 7B depict graphs illustrating the carbon monoxidepurification performance with respect to the rhodium-supported catalystsof Example 6 and Comparative Example 4; FIG. 7A is the results of thetemperature rising process and FIG. 7B is the results of the temperaturedropping process.

FIG. 8A and FIG. 8B depict graphs illustrating the carbon monoxidepurification performance with respect to the rhodium cluster-supportingcatalysts of Examples 7 to 10; FIG. 8A is the results of the temperaturerising process and FIG. 8B is the results of the temperature droppingprocess.

FIG. 9 is a diagrammatic view of the apparatus used in the preparationof clusters by a method of reduction in liquid.

FIG. 10 is a graph illustrating the fluorescence spectra (excitationwavelength: 350 nm) with respect to samples prepared in Examples 11 and12 in which clusters are prepared by a method of reduction in liquid,and copper ion-exchanged zeolite carrier particles as a referencesample.

FIG. 11 is a graph illustrating the fluorescence spectrum (excitationwavelength: 350 nm) and the fluorescence spectra (fluorescence monitorwavelength: 440 nm, 520 nm) with respect to Example 11 in which clustersare prepared by a method of reduction in liquid.

FIG. 12 is a graph illustrating the fluorescence spectra of the rhodiumcluster-supporting catalysts of Example 13 and Comparative Example 5prepared by a method of laser ablation in liquid and an ionexchange-reduction method, respectively.

FIG. 13A and FIG. 13B depict graphs illustrating the results of a testby hydrogen temperature-programmed reduction method (H₂-TPR) withrespect to the rhodium cluster-supporting catalyst of Example 15prepared by a method of laser ablation in liquid.

FIG. 14A and FIG. 14B depict graphs illustrating the results of a H₂-TPRtest with respect to the rhodium cluster-supporting catalyst ofComparative Example 7 prepared by an ion exchange-reduction method.

FIG. 15 is a graph illustrating the results of a carbon monoxideoxidation test with respect to the palladium cluster-supportingcatalysts of Example 16 prepared by a method of laser ablation in liquidand Comparative Example 8 prepared by an ion exchange-reduction method.

FIG. 16 is a graph illustrating the results of a carbon monoxideoxidation test with respect to the platinum cluster-supporting catalystsof Example 17 prepared by a method of laser ablation in liquid andComparative Example 9 prepared by an ion exchange-reduction method.

FIG. 17A and FIG. 17B depict graphs illustrating the results of a nitricoxide temperature-programmed desorption test with respect to thesupported copper-cluster catalysts of Example 18 prepared by a method oflaser ablation in liquid and Comparative Example 10 prepared by an ionexchange-reduction method.

FIG. 18 is a graph illustrating the fluorescence spectra with respect tothe platinum cluster-supporting catalysts of Example 19 prepared by apositive-negative inversion method and Comparative Example 11 preparedby an ion exchange-reduction method.

FIG. 19A and FIG. 19B depict graphs illustrating the nitric oxidereduction test results (gas composition) with respect to the rhodiumcluster-supporting catalysts of Example 20 prepared by a method of laserablation in liquid and Comparative Example 12 prepared by an ionexchange-reduction method.

FIG. 20 is a graph illustrating the nitric oxide reduction test resultswith respect to the rhodium cluster-supporting catalysts of Example 20prepared by a method of laser ablation in liquid and Comparative Example12 prepared by an ion exchange-reduction method.

FIG. 21 is a graph illustrating the results of an oxygen oxidationreaction test of adsorbed carbon monoxide after cleaning treatment withrespect to the platinum cluster-supporting catalyst of Example 21prepared by a method of laser ablation in liquid and the rhodiumcluster-supporting catalysts of Example 22 prepared by a method of laserablation in liquid, and the results of an oxygen oxidation reaction testof adsorbed carbon monoxide before and after cleaning treatment withrespect to a general three-way catalyst.

MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention is described in detail below.The present invention is not limited to the following embodiments andcan be implemented by making various modifications therein withoutdeparting from the scope of the gist of the present invention.

<<Production Method of Cluster-Supporting Catalyst>>

In the method of the present invention for producing acluster-supporting catalyst, a cluster-supporting catalyst,particularly, the cluster-supporting catalyst of the present invention,including porous carrier particles having acid sites, and catalyst metalclusters supported within the pores of the porous carrier particles, isproduced.

This method includes: providing a dispersion liquid containing adispersion medium and porous carrier particles dispersed in thedispersion medium; and in the dispersion liquid, forming catalyst metalclusters having a positive charge, and supporting the catalyst metalclusters on the acid sites within the pores of the porous carrierparticles through an electrostatic interaction. The method may furtherinclude drying and firing the porous carrier particles having supportedthereon the catalyst metal clusters.

According to the method of the present invention, in a dispersion liquidin which porous carrier particles is present, catalyst metal clustersare formed and the formed catalyst metal clusters are supported withinthe pores of the porous carrier particles, whereby catalyst metalclusters having a controlled size, particularly, catalyst metal clustershaving a relatively uniform size, can be supported within the pores ofthe porous carrier particle. The size of the catalyst metal clusters canbe controlled by adjusting the conditions for forming the catalyst metalclusters in the dispersion liquid.

It is considered that according the method of the present invention,when the catalyst is subjected to thermal endurance, the catalyst metalclusters are prevented from becoming unstable and being readilyaggregated and a cluster-supporting catalyst having improved heatresistance can thereby be obtained. On the contrary, in the case ofsupporting a catalyst metal ion within the pores through ion exchangeand after drying, reducing the catalyst metal ion to form catalyst metalclusters within the pore, it is believed that because the clusterssupported within the pores have a relatively non-uniform size, theclusters are likely to become unstable and aggregate when subjected tothermal endurance.

In the method of the present invention, catalyst metal clusters can besupported within the pores of the porous carrier particles through anelectrostatic interaction.

The electrostatic interaction enables the catalyst metal clusters havinga positive charge to be supported on the acid sites within the pores ofthe porous carrier particles having a negative charge.

<Catalyst Metal>

The catalyst metal constituting the catalyst metal clusters may be anymetal or half-metal usable as a catalyst in the intended application.The catalyst metal is selected, for example, from the group consistingof gold, silver, platinum, palladium, rhodium, iridium, ruthenium,titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper,zinc, molybdenum, tungsten, rhenium, silicon, germanium, and acombination thereof.

The cluster-supporting catalyst obtained by the method of the presentinvention can stably maintain the catalytic activity of the catalystmetal, and the catalytic activity of the catalyst metal can thereby beprovided while reducing the use amount of the catalyst metal.Accordingly, the method of the present invention is effectiveparticularly when an expensive catalyst metal is used, for example, whena catalyst metal selected from the group consisting of platinum,palladium, rhodium, iridium ruthenium and a combination thereof is usedas the catalyst metal.

Incidentally, in view of catalytic activity, the catalyst metal ispreferably particles having a fine particle diameter. As described belowregarding the method of the present invention for judging the metalparticle size, the catalyst metal particles can be confirmed to have afine particle diameter by utilizing a phenomenon that when the metalparticles have a particle diameter of 1 nm or less, particularly, whenthe metal particles are of a cluster size, the metal particles emitsfluorescence upon irradiation with excitation light.

<Porous Carrier Particle>

The porous carrier particles may be any porous carrier particles usablein the intended application, and may be, for example, particles of amaterial selected from the group consisting of a microporous material, amesoporous material, a macroporous material, and a combination thereof.

The microporous material means a porous material in which a maximum peakin the volume-based pore diameter distribution is present in the rangefrom more than 0.1 nm to 2 nm; the mesoporous material means a porousmaterial in which a maximum peak in the pore diameter distribution aboveis present in the range from more than 2 nm to 50 nm; and themacroporous material means a porous material in which a maximum peak inthe pore diameter distribution above is present in the range of morethan 50 nm. With respect to the present invention, the volume-based porediameter distribution is a value by the nitrogen adsorption method andcan be obtained according to BET equation (adsorption isotherm equation)by using, for example, an automatic specific surface area/porosimetryanalyzer Tristar II 3020 Series (Shimadzu Corporation).

Specifically, for example, the microporous material includes a materialselected from the group consisting of activated carbon, zeolite, claysilicate, and a combination thereof. The zeolite may be a zeolitederivative or a heteroelement-doped zeolite, and the dissimilar elementdoped in the heteroelement-doped zeolite may be, for example, an elementselected from the group consisting of boron, iron, germanium, gallium,lanthanum, titanium, and a combination thereof.

In addition, the zeolite may be arbitrary zeolite usable in the intendedapplication and includes, for example, A-type (code: LTA), ferrieritetype (code: FER), MCM-22 type (code: MWW), ZSM-5 type, silicalite type(code: MFI), mordenite type (code: MOR), L-type (code: LTL), Y-type andX-type (code: FAU), beta-type (code: BEA), SSZ-type (code: CHA), and acombination thereof.

The mesoporous material includes a material selected from the groupconsisting of porous ionic crystal, mesoporous silica, mesoporoustitania, and a combination thereof, which is, for example, MCM-41 andFSM-16. In addition, the mesoporous material includes PorousCoordination Polymer (PCP)/Metal-Organic Framework (MOF), which may havea pore diameter ranging from 0 to several nm.

The macroporous material includes a macroporous metal oxide, amacroporous semiconductor oxide, and a combination thereof, for example,macroporous titanium oxide, macroporous tin oxide, and macroporous zincoxide.

The porous carrier particles are preferably a porous carrier having acidsites (i.e., electron-rich sites or sites having negative charges) inthe pore, for example, zeolite, in order to stably support the catalystmetal within the pores and/or successfully bring out the activity of thecatalyst metal supported.

In the method of the present invention for producing acluster-supporting catalyst, zeolite particles are preferably used asthe porous carrier particles. Because, in the case of using zeoliteparticles as the porous carrier particles, the catalyst metal clustershaving a positive charge can be supported on the acid sites within thepores of the zeolite particles having a negative charge. Accordingly,the zeolite particles preferably have a relatively small zeta potentialand may have a zeta potential of, for example, −50 mV or less, −70 mV orless, −90 mV or less, or −100 mV or less. For the same reason, thezeolite particles preferably has a relatively large number of acidsites, i.e., has a relatively small Si/Al ratio, and may have an Si/Alratio of, for example, 500 or less, 300 or less, 100 or less, or 50 orless.

In the method of the present invention, the dispersion liquid can beprovided by pulverizing the porous carrier particle and dispersing thepulverized porous carrier particles in the dispersion medium.

In this case, since the porous carrier particles are previouslypulverized, the catalyst metal clusters can be accelerated to besupported within the pores of the porous carrier particle. Incidentally,such a pulverized porous carrier particles have sometimes becomeamorphous, and the porous carrier particles may therefore berecrystallized, if desired, by annealing before or after the catalystmetal clusters are supported.

<Dispersion Medium of Dispersion Liquid>

As the dispersion medium of the dispersion liquid, any dispersion mediumcapable of drawing the catalyst metal clusters into the pores of theporous carrier particles through an electrostatic interaction betweenthe catalyst metal clusters and the acid sites of the porous carrierparticles can be used.

In this connection, in the case where the catalyst metal clusters aresupported within the pores of the porous carrier particles through anelectrostatic interaction, the dispersion medium can be selected so thatthe surface of the catalyst metal clusters can have a positive chargeand the acid sites within the pores of the porous carrier particles canhave a negative charge. Accordingly, in order to accelerate the catalystmetal clusters to be supported within the pores of the porous carrierparticles, the zeta potential and/or the ionization rate of the catalystmetal clusters and/or the porous carrier particles can be adjusted byadjusting the pH of the dispersion medium and/or adding a salt to thedispersion medium.

As regards this, the surface potentials of the catalyst metal clustersand the porous carrier particles cannot be directly measured but can beknown indirectly by measuring the zeta potential (interfacialelectrokinetic potential).

For example, the zeta potential of platinum cluster greatly depends onpH and when the pH is 8 or less, the zeta potential slightly increasesas the pH decreases. This is considered to occur because the platinumatom on the surface of the platinum cluster has been partially oxidizedand the platinum atom on a part of the platinum cluster surface isprotonated into Pt—H⁺ the moment the oxidized platinum atom enters thePt—OH state along with decrease in pH, as a result, the positive chargedensity increases, leading to an increase in the zeta potential.

On the other hand, when the pH is more than 8, the zeta potential of theplatinum cluster rapidly decreases as the pH increases. This isconsidered to occur because the platinum atom oxidized along withincrease in pH becomes Pt—O and furthermore, the platinum clustersurface is partially deprotonated, as a result, the positive chargedensity decreases, leading to a decrease in the zeta potential.

In the case of using an electrostatic interaction, the dispersion mediummay be aqueous or nonaqueous, but it is generally preferable to use anonaqueous dispersion medium, for example, an organic solvent. Because,if an aqueous dispersion medium is used, stabilization of the catalystmetal clusters occurs in the dispersion medium due to the highdielectric constant of water (dielectric constant: 80), i.e., the highpolarity, and the catalyst metal clusters may thereby not besufficiently supported within the pores of the porous carrier particle.

On the other hand, in the case of using a dispersion medium having arelatively low polarity, i.e., a dispersion medium having a relativelylow dielectric constant, the catalyst metal clusters are not stabilizedin the dispersion medium and can be supported within the pores of theporous carrier particles through an electrostatic interaction andstabilized there.

Accordingly, as the dispersion medium, a dispersion medium having alower dielectric constant than that of water (dielectric constant: 80),for example, a dispersion medium having a dielectric constant of 50 orless, 40 or less, 30 or less, 25 or less, or 20 or less, can be used.Specifically, acetone (dielectric constant: 20), 2-propanol (dielectricconstant: 18), ethanol (dielectric constant: 25), methanol (dielectricconstant: 32), carbon tetrachloride (dielectric constant: 2.2), etc. canbe used as the dispersion medium.

<Formation of Catalyst Metal Cluster>

The catalyst metal cluster, particularly, the catalyst metal clustershaving a positive charge, can be formed by any method in the dispersionmedium. The method for forming such catalyst metal clusters includesmethods such as method of laser ablation in liquid, method of microwaveablation in liquid, method of plasma abrasion in liquid,positive-negative inversion method, and method of reduction in liquid(liquid-phase reduction method).

The method of laser ablation in liquid, method of microwave ablation inliquid and method of plasma ablation in liquid are a method of formingcatalyst metal clusters by irradiating a catalyst metal target disposedin a dispersion medium with laser, microwave or plasma.

In the positive-negative inversion method, first, negatively chargedzeolite and a metal ion source having a negative charge are caused to bepresent together in a solution, particularly, in an aqueous solution.Specifically, for example, in the case of using H₂[PtCl₆], platinum iscaused to be present as a negative ion of [PtCl₆]². In this state, anion exchanger is not produced due to repulsive force between negativecharges. A pulsed laser is converged and introduced into this solution.According to this introduction, a plasma is generated in the laser focusregion to produce various chemical species (a metal ion from which aligand is removed, a plasma metal ion produced after electron detachmentof a negative metal ion source, etc.) from the metal ion source, andfurthermore, a neutral metal atom forms an aggregate with a positivemetal ion to produce a positively charged metal clusters. The positivelycharged metal cluster produced in this way is supported on the acidsites of zeolite through an electrostatic interaction.

The method of reduction in liquid is a method of forming catalyst metalclusters by reducing the catalyst metal ion with use of a reducingagent.

In the method of reduction in liquid, any reducing agent capable ofreducing the catalyst metal ion in liquid can be used. Specifically, forexample, a hydride reducing agent such as sodium borohydride, and analcohol such as propanol, can be used as the reducing agent. Inaddition, in the method of reduction in liquid, it is also preferable touse a dispersion medium stable to the reducing agent used, which is adispersion medium capable of dissolving a metal salt as a catalyst metalion supply source and the reducing agent. Accordingly, the same compoundcan be used for both the reducing agent and the dispersant, for example,an alcohol can be used for both the reducing agent and the dispersant.

In the method of reduction in liquid, the reduction of the catalystmetal ion may be promoted by optionally using microwave or plasma inliquid, in addition to the reducing agent,

Incidentally, in order to form catalyst metal clusters having acontrolled size, e.g., catalyst metal clusters having a relativelyuniform size, for example, in the method of reduction in liquid, uniformreduction of the catalyst metal ion can be promoted in the dispersionliquid by optionally using microwave or plasma in liquid, in addition tothe reducing agent.

<<Cluster-Supporting Catalyst>>

The cluster-supporting catalyst of the present invention includes porouscarrier particles having acid sites, and catalyst metal clusterssupported within the pores of the porous carrier particle.

The cluster-supporting catalyst of the present invention has excellentlow-temperature activity even after thermal endurance treatment, andthis means that the cluster in the porous carrier is stably maintaineddespite being subjected to thermal endurance treatment. Although not tobe bound by theory, the stability of the clusters are believed to beattributable to the fact that, for example, the clusters having apositive charge are stably fixed to the acid sites having a negativecharge, and/or the clusters supported within the pores have a relativelyuniform size.

On the other hand, it is considered that in the case of supporting thecatalyst metal ion within the pores by ion exchange and after drying,reducing the catalyst metal ion to thereby form catalyst metal clusterswithin the pore, the clusters supported within the pores have arelatively non-uniform size and is thereby likely to become unstable andaggregate when thermal endurance treatment is performed.

For example, in the cluster-supporting catalyst of the presentinvention, catalyst metal clusters having a positive charge, which isformed in a dispersion liquid containing a dispersion medium and porouscarrier particles dispersed in the dispersion medium, is supported onthe acid sites within the pores of the porous carrier particles throughan electrostatic interaction, and the catalyst metal clusters supportedwithin the pores of the porous carrier particles are thereby formed.

In addition, for example, the cluster-supporting catalyst of the presentinvention satisfies any of the following properties (a) to (d).

(a) The catalyst metal is rhodium and satisfies at least one of thefollowing (a1) to (a3).

(a1) When the cluster-supporting catalyst is subjected to a firstthermal endurance treatment for rhodium, then to an oxygen adsorptionpretreatment and further to a test by hydrogen temperature-programmedreduction method, the peak of reaction between hydrogen supplied andoxygen adsorbed to the cluster-supporting catalyst is present in thetemperature range of 150° C. or less. The reaction peak may be present,for example, in the temperature range of 100° C. or more.

The “first thermal endurance treatment for rhodium” is a treatment ofsubjecting the cluster-supporting catalyst to heating for 2 hours in anatmosphere at 800° C. containing 20 vol % of oxygen and the balancehelium and then to heating for 1 hour in an atmosphere at 800° C.containing 0.5 vol % of hydrogen and the balance helium.

The “oxygen adsorption pretreatment” is a treatment of adsorbing oxygento the cluster-supporting catalyst at 30° C. for 1 hour in an oxygenatmosphere and removing excess oxygen at 500° C. for 1 hour in a heliumatmosphere.

The “test by hydrogen temperature-programmed reduction method” is a testof flowing a reducing gas containing 0.5 vol % of hydrogen and thebalance helium at a spatial velocity of 10,000 h⁻¹ for thecluster-supporting catalyst while raising the temperature at a rate of10° C./min from 20° C.

(a2) When the cluster-supporting catalyst is subjected to a secondthermal endurance treatment for rhodium and then to a nitric oxidereduction test, (i) the reaction temperature at the time of half of thenitric oxide supplied being reduced into nitrogen satisfies at leasteither one of 300° C. or less in the temperature rising process and 270°C. or less in the temperature dropping process, and/or (ii) the numberof molecules of the nitric oxide molecule when nitric oxide can bereduced by one rhodium atom at a temperature of 250° C. in thetemperature dropping process is 0.005 molecules/sec or more. Thereaction temperature may be, for example, 290° C. or less, or 280° C. orless, and may be 250° C. or more, 260° C. or more, or 270° C. or more,in the temperature rising process. The reaction temperature may be, forexample, 260° C. or less and may be 240° C. or more, or 250° C. or more,in the temperature dropping process. The number of molecules of thenitric oxide molecule may be 0.006 molecules/sec or more, or 0.007molecules/sec or more, and may be 0.010 molecules/sec or less, 0.009molecules/sec or less, or 0.008 molecules/sec or less.

The “second thermal endurance treatment for rhodium” is a treatment ofheating the cluster-supporting catalyst for 1 hour in an atmosphere at800° C. containing 8 vol % of oxygen, 0.3 vol % of carbon monoxide andthe balance helium.

The “nitric oxide reduction test” is a test of flowing a model gascontaining 0.1 vol % of ¹⁵NO, 0.65 vol % of CO and the balance helium ata spatial velocity of 10,000 h⁻¹, performing a temperature risingprocess of raising the temperature at a rate of 10° C./min to 800° C.from room temperature, and then performing a temperature droppingprocess of lowering the temperature to room temperature.

(a3) When the cluster-supporting catalyst is subjected to a cleaningtreatment and then to an adsorbed carbon monoxide oxidation test, thepeak of reaction between carbon monoxide adsorbed to thecluster-supporting catalyst and oxygen in the atmosphere is present inthe temperature range of 200° C. or less. The peak may be a maximumpeak, for example, in the temperature range of 50 to 700° C. Inaddition, the peak may be present in the temperature range of 100° C. ormore.

The “cleaning treatment” consists of the following steps (i) to (iv):

(i) putting the catalyst at a concentration of 4 mass % in an aqueous 1M sodium chloride solution, followed by stirring at 80° C. for 10 days,

(ii) after (i) above, rinsing the catalyst with ion-exchanged water,

(iii) after (ii) above, putting the catalyst at a concentration of 4mass % in an aqueous solution containing 6 mass % of polyoxyethylenesorbitan monolaurate, 0.25 M trisodium ethylenediaminetetraacetate, and0.01 M sodium borohydride, followed by stirring at 80° C. for 10 days,and

(iv) after (iii) above, rinsing the catalyst with ion-exchanged water.

The adsorbed carbon monoxide oxidation test is a test of adsorbingcarbon monoxide to the cluster-supporting catalyst by holding thecluster-supporting catalyst at 800° C. for 1 hour in an atmospherecontaining 500 ppm by volume of carbon monoxide and the balance helium,and thereafter oxidizing the carbon monoxide adsorbed to thecluster-supporting catalyst into carbon dioxide with oxygen by heatingthe cluster-supporting catalyst having adsorbed thereto carbon monoxideat a rate of 10° C./min to 800° C. in an atmosphere containing 10 vol %of oxygen and the balance helium.

(b) The catalyst metal is palladium and

when the cluster-supporting catalyst is subjected to a thermal endurancetreatment for palladium and then to a carbon monoxide purification testfor palladium, the number of molecules of the carbon monoxide moleculecapable of being oxidized to carbon dioxide molecule by one palladiumatom is 0.004 molecules/sec or more. The number of molecules of thecarbon monoxide molecule may be, for example, 0.005 molecules/sec ormore, 0.006 molecules/sec or more, or 0.007 molecules/sec or more, andmay be 0.020 molecules/sec or less, 0.015 molecules/sec or less, or0.010 molecules/sec or less.

The “thermal endurance treatment for palladium” is a treatment ofheating the cluster-supporting catalyst for 10 hours in an atmosphere at800° C. containing 20 vol % of oxygen and the balance helium.

The “carbon monoxide purification test for palladium” is a test offlowing a model gas containing 0.3 vol % of carbon monoxide, 8.0 vol %of oxygen and the balance helium at a spatial velocity of 10,000 h⁻¹ forthe cluster-carried catalyst, performing a temperature rising process ofraising the temperature at a rate of 10° C./min to 800° C. from roomtemperature, then performing a temperature dropping process of loweringthe temperature to room temperature, and measuring the catalyticactivity at a temperature of 100° C. in the temperature droppingprocess.

(c) The catalyst metal is platinum and satisfies at least one of thefollowing (c1) and (c2).

(c1) When the cluster-supporting catalyst is subjected to a thermalendurance treatment for platinum and then to a carbon monoxidepurification test for platinum, the number of molecules of the carbonmonoxide molecule capable of being oxidized to carbon dioxide moleculeby one platinum atom is 0.00015 molecules/sec or more. The number ofmolecules of the carbon monoxide molecule may be, for example, 0.00015molecules/sec or more, or 0.00017 molecules/sec or more, and may be0.00030 molecules/sec or less, or 0.00025 molecules/sec or less.

The “thermal endurance treatment for platinum” is a treatment of heatingthe cluster-supporting catalyst for 10 hours in an atmosphere at 800° C.containing 20 vol % of oxygen and the balance helium.

The “carbon monoxide purification test for platinum” is a test offlowing a model gas containing 0.3 vol % of carbon monoxide, 8.0 vol %of oxygen and the balance helium at a spatial velocity of 10,000 h⁻¹ forthe cluster-carried catalyst, performing a temperature rising process ofraising the temperature at a rate of 10° C./min to 800° C. from roomtemperature, then performing a temperature dropping process of loweringthe temperature to room temperature, and measuring the catalyticactivity at a temperature of 60° C. in the temperature dropping process.

(c2) When the cluster-supporting catalyst is subjected to a cleaningtreatment and then to an adsorbed carbon monoxide oxidation test, thepeak of reaction between carbon monoxide adsorbed to thecluster-supporting catalyst and oxygen in the atmosphere is present inthe temperature range of 200° C. or less. The peak may be a maximumpeak, for example, in the temperature range of 50 to 700° C. Inaddition, the peak may be present in the temperature range of 50° C. ormore.

The “cleaning treatment” consists of the following steps (i) to (iv):

(i) putting the catalyst at a concentration of 4 mass % in an aqueous 1M sodium chloride solution, followed by stirring at 80° C. for 10 days,

(ii) after (i) above, rinsing the catalyst with ion-exchanged water,

(iii) after (ii) above, putting the catalyst at a concentration of 4mass % in an aqueous solution containing 6 mass % of polyoxyethylenesorbitan monolaurate, 0.25 M trisodium ethylenediaminetetraacetate, and0.01 M sodium borohydride, followed by stirring at 80° C. for 10 days,and

(iv) after (iii) above, rinsing the catalyst with ion-exchanged water.

The “adsorbed carbon monoxide oxidation test” is a test of adsorbingcarbon monoxide to the cluster-supporting catalyst by holding thecluster-supporting catalyst at 800° C. for 1 hour in an atmospherecontaining 500 ppm by volume of carbon monoxide and the balance helium,and thereafter oxidizing the carbon monoxide adsorbed to thecluster-supporting catalyst into carbon dioxide with oxygen by heatingthe cluster-supporting catalyst having adsorbed thereto carbon monoxideat a rate of 10° C./min to 800° C. in an atmosphere containing 10 vol %of oxygen and the balance helium.

(d) The catalyst metal is copper and

when the cluster-supporting catalyst is subjected to a nitric oxidetemperature-programmed desorption test, the maximum peak in the range of200 to 400° C. is present in the range of 200 to 300° C. The position ofthe maximum peak may be, for example, at 220° C. or more, 240° C. ormore, or 260° C. or more, and at 300° C. or less, 290° C. or less, or280° C. or less.

The “nitric oxide temperature-programmed desorption test” is a test ofadsorbing nitric oxide to the supported catalyst through heating at 800°C. for 1 hour in an atmosphere containing 10 vol % of oxygen and thebalance helium, heating at 800° C. for 30 minutes in an atmospherecontaining 100 vol % of helium, lowering of the ambient temperature to25° C., holding for 1 hour in an atmosphere containing 500 ppm by volumeof nitric oxide and the balance helium, and holding for 1 hour in anatmosphere containing 100 vol % of helium, and thereafter heating thesupported catalyst having adsorbed thereto nitric oxide at a temperaturerise rate of 10° C./min to 800° C. in an atmosphere containing 100 vol %of helium.

The “peak” as used herein means a portion where when a graph of gasconcentration is obtained, the ratio (S/N ratio) between peak signal (S)and noise (N) has a minimum value of 2.0 or more, 3.0 or more, 4.0 ormore, or 5.0 or more, or a maximum value. Specifically, the noise (N)can be specified as a concentration variation range at 30° C. selectedsuch that the concentration variation in a place around the peak, whichis not the peak, becomes minimum near the peak in the graph of gasconcentration. The peak signal (S) can be measured as the distance fromthe center value of the noise portion near the peak.

In the cluster-supporting catalyst of the present invention, thecatalyst metal supporting rate in pores may be 10 mol % or more, 20 mol% or more, 30 mol % or more, 40 mol % or more, 50 mol % or more, 60 mol% or more, or 62.5 mol % or more. In addition. the catalyst metalsupporting rate in pores may be less than 100 ml %, 90 mol % or less, 80mol % or less, or 70 mol % or less.

The “cluster” is generally defined as an aggregate of up to severalhundred chemical species, but in the present invention, the“cluster-supporting catalyst” means a catalyst in which a fine catalystmetal including a clusters are supported on the carrier particle.

The “catalyst metal supporting rate in pore” is an indicator of theratio of the catalyst metal entered the pores of the porous carrierparticles to all the catalyst metal supported on the porous carrierparticle.

In the present invention, the “catalyst metal supporting rate in pore”is represented by the formula: B/A.

In the formula above, the number of atoms “A” of the catalyst metalmeans the number of atoms of all the catalyst metal supported on theporous carrier particle. Moe specifically, the number of atoms “A” ofthe catalyst metal means the number of atoms of all the catalyst metalsupported on the porous carrier particles, irrespective of whether thecatalyst metal is present in the state of one atomic ion, in the stateof a cluster, or in the state of a particle larger than a cluster, orwhether the catalyst metal is supported within the pores or supported onthe outer surface.

With respect to the number of atoms “A” of the catalyst metal, thenumber of atoms of the catalyst metal can be measured by dissolving thecatalyst metal and the carrier particles supporting the catalyst metalin an acid, etc., and performing inductively coupled plasma spectrometry(ICP-OES). In this case, as the ICP-OES apparatus, for example, SPS5100or SPS3000 of Hitachi High-Tech Science Corporation can be used.

The number of atoms “B” corresponds of the number of atoms of thecatalyst metal present as clusters and particles within the pores of theporous carrier particles. Specifically, the number of atoms “B” is thenumber of atoms (mol/g) of the catalyst metal, determined by ether thefollowing evaluation standard (B1) or (B2).

The number of atoms of the catalyst metal determined by the evaluationstandard (B1) is the number of atoms (mol/g) of the catalyst metalobtained by subtracting the catalyst metal present as one atomic ion andthe catalyst metal supported on the outer surface of the porous carrierparticles, from the number of atoms of all the catalyst metal supportedon the porous carrier particle.

The evaluation standard (B1) is based on the assumption that: the“catalyst metal present as one atomic ion” is removable on the ionexchange principle; the “catalyst metal supported on the outer surfaceof the porous carrier particle” is removable by cleaning with a reducingagent, an acid, an alkali, an organic compound cleaner, a surfactant,and a chelating agent; and whether the catalyst metal supported on theouter surface could be removed can be confirmed by electron microscopeobservation. In other words, the number of atoms “B” of the catalystmetal determined according to (B2) corresponds to the amount of thecatalyst metal present within the pores of the porous carrier particle.

The number of atoms of the catalyst metal determined by the evaluationstandard (B2) is the number of atoms (mol/g) of the catalyst metalsupported in the cluster-supporting catalyst after the followingtreatments (i) to (iv):

(i) putting the cluster-supporting catalyst at a concentration of 4 mass% in an aqueous 1 M sodium chloride solution, followed by stirring at80° C. for 10 days,

(ii) after (i) above, rinsing the cluster-supporting catalyst withion-exchanged water,

(iii) after (ii) above, putting the cluster-supporting catalyst at aconcentration of 4 mass % in an aqueous solution containing 6 mass % ofpolyoxyethylene sorbitan monolaurate (for example, Tween 20 (trademark)of Tokyo Chemical Industry Co., Ltd.), 0.25 M trisodiumethylenediaminetetraacetate, and 0.01 M sodium borohydride, followed bystirring at 80° C. for 10 days, and

(iv) after (iii) above, rinsing the catalyst with ion-exchanged water.

In the operation of the evaluation standard (B2), the number of atoms ofthe catalyst metal means the number of atoms of the catalyst metalsupported within the pores of the porous carrier particles after thecatalyst metal present as one atomic ion within the pores or on theouter surface of the porous carrier particles is dissolved in a saltsolution and thereby removed (treatments (i) and (ii)) and the catalystmetal present on the outer surface of the porous carrier particleswithout entering the pores of the porous carrier particles is removedusing a nonionic surfactant, a chelating agent, and a reducing agent(treatments (iii) and (iv)). In other words, the number of atoms “B” ofthe catalyst metal determined by the evaluation standard (B2)corresponds to the amount of the catalyst metal present within the poresof the porous carrier particle.

Catalyst metal clusters larger than the pores of the porous carrierparticles are of course prevented from entering the pores of the porouscarrier particle. Accordingly, the catalyst metal being present withinthe pores of the porous carrier particles means that the size of thecatalyst metal clusters are smaller than the size of the pores of theporous carrier particle. Specifically, the catalyst metal being present,for example, within the pores of a microporous material, i.e., a porousmaterial mainly having a pores of 2.0 nm or less, suggests that thecatalyst metal clusters are clusters having a particle diameter of 2.0nm or less, particularly, 1.0 nm or less. In other words, the number ofatoms “B” of the catalyst metal present within the porous carrierparticles as a microporous material corresponds to the amount of thecluster within the pore.

With respect to the number of atoms “B” of the catalyst metal, thenumber of atoms of the catalyst metal can be measured by dissolving thecatalyst metal and the carrier particles supporting the catalyst metalin an acid, etc., and performing inductively coupled plasma spectrometry(ICP-OES). In this case, as the ICP-OES apparatus, for example, SPS5100or SPS3000 of Hitachi High-Tech Science Corporation can be used.

In the cluster-supporting catalyst of the present invention, thecatalyst metal is present in a fine state, whereby high and/or inherentcatalytic activity can be provided. Furthermore, in thecluster-supporting catalyst of the present invention, the catalyst metalsupporting rate in pore, i.e., the ratio of the catalyst metal enteredthe pores of the porous carrier particles to all the catalyst metalsupported on the porous carrier particles, is large, whereby thecatalytic activity of the catalyst metal can be stably maintained and/orhigh efficiency can be brought out.

Accordingly, the cluster-supporting catalyst of the present inventioncan be preferably used, for example, as an exhaust gas purificationcatalyst, a catalyst for liquid-phase compound synthesis reaction, acatalyst for gas-phase synthesis reaction or a catalyst for fuel cellreaction, particularly, as an exhaust gas purification catalyst.

On the other hand, even when a large amount of fine catalyst metal issupported on a carrier particles, if the catalyst metal is supported onthe outer surface of the carrier particles but not within the pores ofthe carrier particles, the catalyst metal moves and sinters with eachother during use of the catalyst, and the activity can thereby be hardlymaintained.

<Catalyst Metal>

As to the catalyst metal usable in the cluster-supporting catalyst ofthe present invention, the description regarding the method of thepresent invention may be referred to.

<Porous Carrier Particle>

As to the porous carrier particles usable in the cluster-supportingcatalyst of the present invention, the description regarding the methodof the present invention may be referred to.

<<Catalyst Device>>

The catalyst device of the preened invention includes thecluster-supporting catalyst of the present invention, a substratesupporting the cluster-supporting catalyst, and optionally, a vesselholding the substrate.

In the catalyst device of the present invention, a honeycomb substrate,particularly, a cordierite-made honeycomb substrate, can be used as thesubstrate. Furthermore, in the catalyst device of the present invention,a vessel made of a metal such as stainless steel can be used as theoptional vessel.

<<Method for Evaluating Size of Catalyst Metal Particles Supported inSupported Catalyst>>

The method of the present invention for evaluating the size of catalystmetal particles supported in a supported catalyst includes the followingsteps:

providing a dispersion liquid having dispersed therein the supportedcatalyst, and

evaluating the size of the catalyst metal particles in the supportedcatalyst based on the presence or absence of fluorescence emitted fromthe supported catalyst by irradiating the dispersion liquid withexcitation light.

The supported catalyst is a catalyst including porous carrier particleshaving acid sites, and catalyst metal clusters supported on the porouscarrier particle

The method of the present invention utilizes a phenomenon that when themetal particles have a particle diameter of 1 nm or less, particularly,when the metal particles are of cluster size, the metal particles emitfluorescence upon irradiation with excitation light. More specifically,in the method of the present invention, whether the catalyst metalparticles of the supported catalyst are in the size of a particleemitting fluorescence, i.e., whether the catalyst metal particles have aparticle diameter of 1 nm or less, particularly, whether the catalystmetal particles are of cluster size, can be evaluated in the state ofthe catalyst metal particles being supported on the porous carrierparticle.

In the method of the present invention, as the dispersion medium neededto disperse the supported catalyst, any dispersion medium enabling thesupported catalyst to be dispersed therein and not reacting inparticular with the supported catalyst can be used.

As the excitation light, a long-wavelength ultraviolet ray, for example,an ultraviolet ray having a wavelength of about 350 nm, can be used. Afluorescent spectrometer can be used for confirming the presence orabsence of fluorescence. Furthermore, in a simplified manner, thepresence or absence of fluorescence can be confirmed with an eye byusing black light as the light source of excitation light.

Incidentally, in the method above, the catalyst metal particles arepreferably subjected to a reduction treatment before evaluating the sizeof the catalyst metal particle. While metal particles having a particlediameter of 1 nm or less emit fluorescence, a metal oxide particleshaving the same size do not emit fluorescence. On the other hand,according to the reduction treatment, a metal oxide particle can bereduced to emit fluorescence, and the size of the catalyst metalparticles can thereby be evaluated more accurately.

The present invention is described in greater detail below by referringto Examples, but needless to say, the scope of the present invention isnot limited by these Examples.

EXAMPLES Examples 1 and 2 and Comparative Example 1

In Examples 1 and 2 and Comparative Example 1, gold clusters were formedby a laser ablation method of gold target in acetone, and the goldclusters were supported on carrier particle to prepare a supportedgold-cluster catalyst. The catalysts obtained in Examples 1 and 2 andComparative Example 1 were evaluated for the fluorescence spectrum. Thecatalyst of Example 1 was evaluated also for the supporting rate inpore.

Example 1

As illustrated in FIG. 1A, acetone 11 as a dispersion medium havingdispersed therein carrier particles (not shown) was put in a vessel 13,a plate 12 of gold was placed in acetone 11, the plate 12 of gold inacetone 11 was irradiated with a laser 15 through a lens 14, and a goldcluster 16 was formed in the acetone by laser ablation. The thus-formedgold cluster 16 took on a positive charge and therefore, as illustratedin FIG. 1B, was electrically drawn to sites having a negative charge,i.e., acid sites, of the carrier particles of zeolite carrier particles20, and supported there.

The laser light was the basic wave (1,064 nm, 10 Hz) of a Nd:YAG laser,and the intensity thereof was 2 W.

The carrier particles supporting the clusters were taken out from theacetone, dried at about 25° C. for about 1 hour, and fired at 300° C.for 2 hours to obtain the supported gold-cluster catalyst of Example 1.

In Example 1, the carrier particles and the laser irradiation time wereas follows:

Carrier particle: ZSM-5 zeolite (MFI) (Si/Al ratio: 1,500)

Laser irradiation time: 2 hours and 45 minutes

Example 2

The supported gold-cluster catalyst of Example 2 was obtained in thesame manner as in Example 1 except that the carrier particles and thelaser irradiation time were as follows:

Carrier particle: ZSM-5 zeolite (MFI) (Si/Al ratio: 1,500)

Laser irradiation time: 12 hours and 30 minutes

Incidentally, since the ablation efficiency differs depending on thesurface state of the carrier particles or the plate of gold, in Example2 and Comparative Example 1, the laser ablation time was adjusted toprovide the same amount of ablation of gold as in Example 1. The amountof ablation of gold was judged from the change in color of thedispersion medium.

Comparative Example 1

The supported gold-cluster catalyst of Comparative Example 1 wasobtained in the same manner as in Example 1 except that the carrierparticles and the laser irradiation time were as follows:

Carrier particle: fumed silica

Laser irradiation time: 30 minutes

<Evaluation: Fluorescence Spectrum>

With respect to the supported gold-cluster catalysts of Examples 1 and 2and Comparative Example 1, the measurement of fluorescence spectrum(excitation wavelength: 350 nm) was performed. FIG. 2A depicts a graphillustrating the evaluation results of fluorescence spectrum normalizedto the intensity per 1 mg of gold. In FIG. 2A, the result as to Example1 is indicated by the spectrum (i), the result as to Example 2 isindicated by the spectrum (ii), and the result as to Comparative Example1 is indicated by the spectrum (iii).

In FIG. 2A, the fluorescence signal at near 400 nm is a spectrum inwhich fluorescent emissions from the gold cluster of about 8-mer wereoverlapped. Accordingly, FIG. 2A indicates that in the supportedgold-cluster catalysts of Examples 1 and 2, particularly, in thesupported gold-cluster catalyst of Example 1, a relatively large amountof a gold cluster around 8-mer is supported on the carrier particle.

In FIG. 2B, for the sake of examination, based on the spectra of FIG.2A, the result as to Example 1 (spectrum (i)) is depicted at 1-foldmagnification, the result as to Example 2 (spectrum (ii)) is depicted at8-fold magnification, and the result as to Comparative Example 1(spectrum (iii)) is depicted at 60-fold magnification.

Compared with the results (spectra (i) and (ii)) as to Examples 1 and 2where the gold clusters were supported on zeolite, the result (spectrum(iii)) as to Comparative Example 1 where the gold clusters weresupported on fumed silica is shifted to the long wavelength side. Thussuggests that the particle diameter of the gold cluster supported onfumed silica of Comparative Example 1 is larger than that of the goldcluster supported on zeolite of Examples 1 and 2. Incidentally, the peaknear 550 nm is derived from Mie scattering by nanoparticles attached tothe carrier particle surface simultaneously with the clusters.

<Evaluation: Catalyst Metal Supporting Rate in Pore>

The supported gold-cluster catalyst of Example 1 was evaluated for thegold, as the catalyst metal, supporting rate in pore. The supportingrate in pores was 62.5 mol %.

Specifically, the catalyst metal supporting rate in pores was determinedas follows:

Catalyst metal supporting rate in pores (mol %)=B/A

A: the number of atoms (mol/g) of all the catalyst metal supported onthe carrier particles,

B: the number of atoms (mol/g) of the catalyst metal supported on thecarrier particles after the following treatments (i) to (iv):

(i) putting the cluster-supporting catalyst at a concentration of 4 mass% in an aqueous 1 M sodium chloride solution, followed by stirring at80° C. for 10 days,

(ii) after (i) above, rinsing the cluster-supporting catalyst withion-exchanged water,

(iii) after (ii) above, putting the cluster-supporting catalyst at aconcentration of 4 mass % in an aqueous solution containing 6 mass % ofpolyoxyethylene sorbitan monolaurate (Tween 20 (trademark) of TokyoChemical Industry Co., Ltd.), 0.25 M trisodiumethylenediaminetetraacetate, and 0.01 M sodium borohydride, followed bystirring at 80° C. for 10 days, and

(iv) after (iii) above, rinsing the catalyst with ion-exchanged water.

<Other Metals>

In Examples 1 to 2 and Comparative Example 1, a gold clusters wereformed by using a gold target. In this connection, as to the followingmetals, it was confirmed that clusters of the metal can be formed in thesame manner as in Example 1 by the method of laser ablation in liquidusing the metal as the target:

Aluminum, silicon, titanium vanadium, chromium, manganese, iron, cobalt,nickel, copper, zinc, gallium, germanium zirconium niobium, silver,rhodium ruthenium, palladium, indium, tin, tantalum, tungsten, iridium,platinum, and cerium.

It was also confirmed that out of these metal clusters, as to copper,silver, rhodium, ruthenium palladium and platinum, fluorescence isobserved upon irradiation with excitation light. In addition, it wasconfirmed that out of these metal clusters, as to copper, silver,rhodium, ruthenium and platinum, fluorescence is observed uponirradiation with excitation light also when clusters formed by the ionexchange-reduction method are supported on a zeolite carrier particle.

Example 3

In Example 3, a supported copper-cluster catalyst in which copperclusters are supported on a zeolite carrier particles was prepared inthe same manner as in Example 1 except that a copper target was used inplace of the gold target and a ZSM-5 zeolite carrier particles (MFI)(Si/Al ratio: 40) were used as the zeolite carrier particle. Theobtained catalyst was evaluated for the fluorescence spectrum.

Unlike the gold, the copper is oxidized in air, and the copper clusterimmediately after preparation was in the oxide state. Accordingly, thesupported copper-cluster catalyst immediately after preparation did notemit fluorescence.

Then, the obtained supported copper-cluster catalyst was heated at 300°C. for 2 hours in a hydrogen atmosphere, effecting a reductiontreatment, and thereafter evaluated for the fluorescence intensity.Consequently, the supported copper-cluster catalyst subjected to areduction treatment exhibited fluorescence. The result of thefluorescence intensity evaluation (excitation wavelength: 350 nm) isillustrated as the spectrum (i) in FIG. 3. In this spectrum (i), thefluorescence of 400 to 500 nm corresponds to the already-reportedfluorescence signals of 8-mer and 9-mer of copper.

Subsequently, the supported copper-cluster catalyst was left standingovernight in an air atmosphere, effecting an oxidation treatment, andagain evaluated for the fluorescence intensity. Consequently, thesupported copper-cluster catalyst left standing in an air atmosphereexhibited fluorescence, though the intensity was weak compared with thatbefore the treatment in an air atmosphere. The result of fluorescenceintensity evaluation is illustrated as the spectrum (ii) in FIG. 3.

Subsequently, the supported copper-cluster catalyst left standing in anair atmosphere was again subjected to the above-described reductiontreatment and again evaluated for the fluorescence intensity.Consequently, the supported copper-cluster catalyst again subjected tothe reduction treatment exhibited the same fluorescence as that beforethe treatment in an air atmosphere. The result of the fluorescenceintensity evaluation is illustrated as the spectrum (iii) in FIG. 3.

The supported copper-cluster catalyst after performing oxidationtreatment and reduction treatment thus exhibited the same fluorescenceas that before these treatments, and this suggests that the copperclusters are held within the pores of the zeolite carrier particles andin turn, a change such as aggregation of the copper clusters are notcaused by these treatments.

Example 4 and Comparative Example 2

In Example 4 and Comparative Example 2, the rhodium cluster-supportingcatalyst (Example 4) and a commercially available exhaust gaspurification catalyst (Comparative Example 2) were evaluated for thecatalytic activity.

Specifically, Example 4 and Comparative Example 2 were conducted asfollows.

Example 4

In Example 4, rhodium clusters were supported on a zeolite carrierparticles in the same manner as in Example 1 except that a rhodiumtarget was used in place of the gold target and a beta-type zeolitecarrier particles (BEA) (Si/Al ratio: 40) were used as the zeolitecarrier particle.

The temperature (T_(CO(50%))) when consuming 50% of the supplied carbonmonoxide was evaluated by repeating an operation of heating 30 mg of theobtained rhodium cluster-supporting catalyst (Rh_(cluster)/BEA) forabout 24 hours in an electric furnace at a heating rate of 12° C./min toa peak heating temperature of 640 to 800° C. from room temperature whileflowing an evaluation gas having the following composition over thecatalyst, and cooling it to room temperature.

Carbon monoxide (CO): 0.3%

Oxygen (O₂): 8%

Helium (He): balance

With respect to the temperature change in the above-described repetitivestep of heating and cooling, as illustrated in FIG. 4, the peak heatingtemperature becomes higher as it goes to the latter half, and a total ofabout 24 hours was spent.

In the repetitive step of heating and cooling, the evaluation wasperformed while raising the evaluation gas temperature to the peakheating temperature, i.e., in the temperature rising process. Similarly,in the repetitive step of heating and cooling, the evaluation wasperformed while lowering the evaluation gas temperature from the peakheating temperature, i.e., in the temperature dropping process.

Comparative Example 2

For reference, with respect to a commercially available exhaust gaspurification catalyst (Rh/Al₂O₃—CeO₂—ZrO₂) as Comparative Example 2, theevaluations in the temperature rising process and the temperaturedropping process were performed in the same manner as in Example 4.

<Evaluation: Durability>

The evaluation results in the temperature rising process and thetemperature dropping process are illustrated respectively in FIGS. 5Aand 5B as the difference between the result of Example 4 and the resultof Comparative Example 2 (T_(CO(50%)) of Example 4)-(T_(CO(50%)) ofComparative Example 2). When the difference above takes a minus value,this indicates that T_(CO(50%)) of Example 4 is lower than T_(CO(50%))of Comparative Example 2, i.e., the low-temperature activity of thecatalyst of Example 4 is excellent. In FIG. 5, the abscissa indicatesthe temperature (the peak temperature in FIG. 4) at which an accelerateddeterioration treatment was performed.

It is understood from FIGS. 5A and 5B that the catalyst of Example 4provides an excellent exhaust gas purification performance relative tothe catalyst of Comparative Example 2 as the peak heating temperaturebecomes higher. This indicates that the catalyst of Example 4 is lesslikely to deteriorate compared with the catalyst of Comparative Example2.

Although not to be bound by theory, it is considered that in thecatalyst of Comparative Example 2, rhodium of various sizes ranging fromthe monoatomic level to the submicrometer level was supported on thecarrier to randomly cause sintering of rhodium particles by heat at thepeak heating temperature and the catalyst was thereby deteriorated,whereas in the catalyst of Example 4, rhodium clusters were stablymaintained within the pores of zeolite and in turn, the catalyst was notdeteriorated due to heat at the peak heating temperature.

Incidentally, the change when the peak heating temperature is 640° C.and 660° C. is a change in the firing process of removing watermolecules adsorbed to zeolite and therefore, the catalytic activityneeds to be evaluated from the change substantially when the peakheating temperature is 700° C. or more.

Example 5 and Comparative Example 3

In Example 5 and Comparative Example 3, a catalyst in which rhodiumclusters are supported on zeolite carrier particles or fumed silicacarrier particles was obtained, and with respect to the obtainedcatalysts, the durability of the catalyst was evaluated.

Specifically, Example 5 and Comparative Example 3 were conducted asfollows.

Example 5

In Example 5, rhodium clusters were supported on zeolite carrierparticles in the same manner as in Example 1 except that a rhodiumtarget was used in place of the gold target and beta-type zeolitecarrier particles (BEA) (Si/Al ratio: 40) were used as the zeolitecarrier particle.

With respect to the obtained rhodium cluster-supporting catalyst(Rh_(cluster)/BEA), the temperature (T_(CO(50%))) when consuming 50% ofthe supplied carbon monoxide was evaluated in the temperature risingprocess and in the temperature dropping process in the same manner as inExample 4.

Comparative Example 3

In Comparative Example 3, rhodium clusters were supported on a fumedsilica particle in the same manner as in Example 1 except that a rhodiumtarget was used in place of the gold target and a fumed silica particlewas used as the carrier particle.

With respect to this rhodium cluster-supporting catalyst(Rh_(cluster)/silica), the temperature (T_(CO(50%))) when consuming 50%of the supplied carbon monoxide was evaluated in the temperature risingprocess and in the temperature dropping process in the same manner as inExample 4.

<Evaluation: Durability>

The evaluation results in the temperature rising process and thetemperature dropping process are illustrated respectively in FIGS. 6Aand 6B as the difference between the result of Example 5 and the resultof Comparative Example 3 (T_(CO(50%)) of Example 5)-(T_(CO(50%)) ofComparative Example 3). When the difference above takes a minus value,this indicates that T_(CO(50%)) of Example 5 is lower than T_(CO(50%))of Comparative Example 3, i.e., the low-temperature activity of thecatalyst of Example 5 is excellent. In FIG. 6, the abscissa indicatesthe temperature (the peak temperature in FIG. 4) at which an accelerateddeterioration treatment was performed.

It is understood from FIGS. 6A and 6B that the catalyst(Rh_(cluster)/BEA) of Example 5 in which rhodium clusters are supportedon beta-type zeolite carrier particles (BEA) has a significantlyexcellent low-temperature activity at all peak heating temperatures,compared with the catalyst (Rh_(cluster)/silica) of Comparative Example3 in which rhodium clusters are supported on a fumed silica carrierparticle.

Although not to be bound by theory, this is considered to beattributable to the fact that since fumed silica used in the catalyst ofComparative Example 3 does not have a pore, the rhodium clusters weresupported only on the surface thereof and in the process of the rhodiumbeing supported on the carrier and/or during the accelerateddeterioration treatment, the rhodium cluster underwent aggregation orgrain growth, i.e., the fact that while the rhodium cluster of thecatalyst of Example 5 is stably maintained within the pores of thezeolite carrier, the rhodium cluster of the catalyst of ComparativeExample 3 is present on the outer surface of the fumed silica carrier.

Example 6 and Comparative Example 4

In Example 6 and Comparative Example 4, a catalyst was obtained bysupporting or not supporting a rhodium cluster on zeolite carrierparticles, and the obtained catalysts were evaluated for the durability.

Specifically, Example 6 and Comparative Example 4 were conducted asfollows.

Example 6

In Example 6, rhodium clusters were supported on zeolite carrierparticles in the same manner as in Example 1 except that a rhodiumtarget was used in place of the gold target and ZSM-5 zeolite carrierparticles (MFI) (Si/Al ratio: 40) were used as the zeolite carrierparticle.

With respect to the obtained rhodium cluster-supporting catalyst(Rh_(cluster)/MFI), the temperature (T_(CO(50%))) when consuming 50% ofthe supplied carbon monoxide was evaluated in the temperature risingprocess and in the temperature dropping process in the same manner as inExample 4.

Comparative Example 4

In Comparative Example 4, rhodium clusters were dispersed in acetone inthe same manner as in Example 1 except that a rhodium target was used inplace of the gold target and carrier particles was not used. Thereafter,at the stage where the rhodium clusters were aggregated to form arhodium cluster aggregate particles, ZSM-5 zeolite carrier particles(MFI) (Si/Al ratio: 40) was added as the zeolite carrier particles tothe acetone to prepare the catalyst (Rh_(particle)/MFI) of ComparativeExample 4 in which the rhodium cluster aggregate particle was supportedon the ZSM-5 zeolite carrier particles (MFI).

With respect to this rhodium aggregate particle-supported catalyst(Rh_(particle)/MFI), the temperature (T_(CO(50%))) when consuming 50% ofthe supplied carbon monoxide was evaluated in the temperature risingprocess and in the temperature dropping process in the same manner as inExample 4.

<Evaluation: Durability>

The evaluation results in the temperature rising process and thetemperature dropping process are illustrated respectively in FIGS. 7Aand 7B as the difference between the result of Example 6 and the resultof Comparative Example 4 (T_(CO(50%)) of Example 6)-(T_(CO(50%)) ofComparative Example 4). When the difference above takes a minus value,this indicates that T_(CO(50%)) of Example 6 is lower than T_(CO(50%))of Comparative Example 4, i.e., the low-temperature activity of thecatalyst of Example 6 is excellent. In FIG. 7, the abscissa indicatesthe temperature (the peak temperature in FIG. 4) at which an accelerateddeterioration treatment was performed.

It is understood from FIGS. 7A and 7B that the catalyst(Rh_(cluster)/MFI) of Example 6 in which rhodium clusters are supportedon a ZSM-5 zeolite carrier particles (MFI) has a significantly excellentlow-temperature activity at all peak heating temperatures, compared withthe catalyst (Rh_(particle)/MFI) of Comparative Example 4 in which arhodium cluster aggregate particles are supported on a ZSM-5 zeolitecarrier particles (MFI).

Although not to be bound by theory, this is considered to beattributable to the fact that since the particle diameter of the rhodiumcluster used in the catalyst of Example 6 is significantly smaller thanthe particle diameter of the rhodium cluster aggregate particle used inthe catalyst of Comparative Example 4, a low-temperature catalyticactivity specific to clusters was exhibited and a relatively largesurface area could be provided for the catalytic reaction.

Examples 7 to 10

In Examples 7 to 10, a catalyst was obtained by supporting a rhodiumcluster on zeolite carrier particles, and the obtained catalysts wereevaluated for the durability.

Specifically, Examples 7 to 10 were conducted as follows.

In Examples 7 to 10, rhodium clusters were supported on zeolite carrierparticles in the same manner as in Example 1 except that a rhodiumtarget was used in place of the gold target and the following carrierparticles were used as the zeolite carrier particle.

Example 7: ZSM-5 zeolite carrier particles (MFI) (Si/Al ratio: 1,500)

Example 8: beta-type zeolite carrier particles (BEA) (Si/Al ratio:1,500)

Example 9: beta-type zeolite carrier particles (BEA) (Si/Al ratio: 40)

Example 10: ZSM-5 zeolite carrier particles (MFI) (Si/Al ratio: 40)

With respect to the obtained rhodium cluster-supporting catalysts ofExamples 7 to 10, the temperature (T_(CO(50%))) when consuming 50% ofthe supplied carbon monoxide was evaluated in the temperature risingprocess and in the temperature dropping process in the same manner as inExample 4.

<Evaluation: Durability>

The evaluation results in the temperature rising process and thetemperature dropping process are illustrated respectively in FIGS. 8Aand 8B as the difference between the results of Examples 7 to 10 and theresult of Comparative Example 2 (commercially available exhaust gaspurification catalyst) (T_(CO(50%)) of Examples 7 to 10)-(T_(CO(50%)) ofComparative Example 2). When the difference above takes a minus value,this indicates that T_(CO(50%)) of Examples 7 to 10 is lower thanT_(CO(50%)) of Comparative Example 2, i.e., the low-temperature activityof the catalysts of Examples 7 to 10 is excellent. In FIG. 8, theabscissa indicates the temperature (the peak temperature in FIG. 4) atwhich an accelerated deterioration treatment was performed.

It is understood from FIGS. 8A and 8B that the catalysts of Examples 7to 10 provides an excellent or equivalent exhaust gas purificationperformance relative to the catalyst of Comparative Example 2 as thepeak heating temperature becomes higher. This indicates that thecatalysts of Examples 7 to 10 are less likely to deteriorate comparedwith the catalyst of Comparative Example 2.

Although not to be bound by theory, this is considered to beattributable to the fact that in the catalyst of Comparative Example 2,rhodium of various sizes ranging from the monoatomic level to thesubmicrometer level was supported on the carrier to randomly causesintering of rhodium particles by heat at the peak heating temperatureand the catalyst was thereby deteriorated, whereas in the catalysts ofExamples 7 to 10, rhodium clusters were stably maintained within thepores of zeolite and in turn, the catalyst was not deteriorated due toheat at the peak heating temperature.

Incidentally, the catalyst of Example 7 showed a poor catalystperformance in the temperature range of up to 800° C. compared with thecatalyst of Comparative Example 2, but it is clearly understood from thecurves of FIGS. 8A and 8B that when the accelerated deteriorationtreatment is further continued, the performance of the catalyst ofExample 7 surpasses the performance of the catalyst of ComparativeExample 2.

Reviewing Examples 7 and 10 using a ZSM-5 zeolite carrier particles(MFI) as the carrier, the catalyst of Example 10 using ZSM-5 zeolitecarrier particles (MFI(40)) having an Si/Al ratio of 40 exhibited goodcatalyst performance, compared with the catalyst of Example 7 usingZSM-5 zeolite carrier particles (MFI(1500)) having an Si/Al ratio of1,500. This is considered to occur because the ZSM-5 zeolite carrierparticles (MFI(40)) having an Si/Al ratio of 40 has a larger number ofacid sites than the ZSM-5 zeolite carrier particles (MFI(1500)) havingan Si/Al ratio of 1,500 and supporting of the rhodium cluster on thezeolite carrier particles was successfully performed by an electrostaticaction.

Reviewing Examples 8 and 9 using a beta-type zeolite as the carrier,similarly to the case of MFI zeolite, the catalyst of Example 9 using abeta-type zeolite carrier particles (BEA(40)) having an Si/Al ratio of40, i.e., a zeolite carrier particles having a relatively large numberof acid sites exhibited good catalyst performance, compared with thecatalyst of Example 8 using beta-type zeolite carrier particles(BEA(1500)) having an Si/Al ratio of 1,500, i.e., zeolite carrierparticles having a relatively small number of acid sites.

However, in the case of using beta-type zeolite carrier particles (BEA),the difference in the catalyst performance due to the different in theSi/Al ratio was not so large as in the case of using ZSM-5 zeolitecarrier particles (MFI). This is considered to occur because thebeta-type zeolite carrier particle (BEA) substantially has a largesurface solid acid strength and the effect of the difference in theamount of acid sites was less likely to develop.

For reference, the zeta potential (indicator of solid acid strength) ofthe zeolite carrier particles used in Examples 7 to 10 is as follows:

Example 7: ZSM-5 zeolite carrier particle (MFI(1500)): −72.7 mV

Example 8: beta-type zeolite carrier particle (BEA(1500)): −96.8 mV

Example 9: beta-type zeolite carrier particle (BEA(40)): −117 mV

Example 10: ZSM-5 zeolite carrier particle (MFI(40)): −87 mV

That is, it is considered that in the case of a beta-type zeolitecarrier particle (BEA), despite a large Si/Al ratio, the zeolitepotential was low and supporting of the rhodium cluster on the zeolitecarrier particle was thereby successfully performed by an electrostaticaction.

In order to confirm this understanding, a rhodium particle was supportedon MFI(40) (zeta potential: −87 mV) and on MFI(1500) (zeta potential:−72.7 mV) by laser ablation in liquid, as a result, coloring of thecarrier caused by the supporting of rhodium particle on the carrierparticle occurred more prominently in MFI(40) than in MFI(1500).

It is understood from this result that in the case of MFI(40) having arelatively small zeta potential, i.e., relatively large acid strength,the rhodium particle was supported on the carrier particle in arelatively successful manner through an electrostatic interactionbetween the rhodium particle and acid sites of the carrier particle.

Examples 11 and 12

In Examples 11 and 12, a supported copper-cluster catalyst was obtainedby a method of reduction in liquid, and the obtained catalysts wereevaluated by means of fluorescence.

Example 11

In Example 11, a zeolite carrier particle was dispersed in 2-propanol toproduce a zeolite carrier particle dispersion liquid, and copper(II)chloride as a copper ion source and sodium borohydride (NaBH₄) as areducing agent were mixed with the dispersion liquid to synthesize acopper cluster in the dispersion liquid. The thus-synthesized coppercluster had a positive charge and was thereby electrically drawn to acidsites of the zeolite carrier particle and supported thereon.

Specifically, copper(II) chloride and sodium borohydride were mixedusing the apparatus illustrated in FIG. 9.

More specifically, a water bath 82 at about 10° C. was disposed on amagnetic stirrer 81, a flask 83 was disposed thereon, a dropping funnel84 was disposed on the flask 83, and the contents 84 a of the droppingfunnel 84 were added dropwise to the contents 83 a of the flask 83 withstirring by means of a stirring bar 81 a. The dropwise addition wasperformed for 1 hour while keeping the temperature by the water bath,and after the completion of dropwise addition, stirring was furtherperformed for 1 hour while keeping the temperature by the water bath.Thereafter, stirring was further performed at room temperature for 2hours, and the contents of the flask were then filtered and fired at atemperature of 300° C. for 2 hours in the atmosphere to obtain thesupported copper-cluster catalyst of Example 11.

The contents 84 a of the dropping funnel 84 and the contents 83 a of theflask 83 in Example 11 are shown together in Table 1 below.

Example 12

The supported copper-cluster catalyst of Example 12 was obtained in thesame manner as in Example 11 except that the contents 84 a of thedropping funnel 84 and the contents 83 a of the flask 83 were changed asshown in Table 1 below.

TABLE 1 Example 11 Example 12 Dropping sodium borohydride 25 μmol copperchloride 12 μmol funnel 2-propanol 10 mL 2-propanol 5 mL Flask copperchloride 12 μmol sodium borohydride 80 μmol zeolite carrier particle 200mg zeolite carrier particle 200 mg 2-propanol 200 mL 2-propanol 200 mL

<Evaluation: Fluorescence Spectrum>

The supported copper-cluster catalysts prepared in Examples 11 and 12and as reference samples, a copper ion-exchanged zeolite carrierparticle and a proton-type zeolite carrier particle were measured forthe fluorescence spectrum at an excitation wavelength of 350 nm. Theresults are illustrated in FIG. 10.

In FIG. 10, the result as to Example 11 is indicated by the spectrum(i), the result as to Example 12 is indicated by the spectrum (ii), theresult as to the copper ion-exchanged zeolite carrier particle as areference sample is indicated by the spectrum (iii), and the result asto the proton-type zeolite carrier particle as a reference sample isindicated by the spectrum (iv).

As understood from FIG. 10, the supported copper-cluster catalystsobtained in Examples 11 and 12, particularly in Example 11, showed apeak at about 440 nm. This peak is considered to be derived from thecopper clusters. In the supported copper-cluster catalyst obtained inExample 11, the peak above is as broad as having a half width of about100 nm and is considered to be derived from the copper clusters.

<Evaluation: Fluorescence Spectrum>

In addition, the supported copper-cluster catalyst obtained in Example11 was measured for the fluorescence spectrum at an excitationwavelength of 350 nm and the excitation spectrum at a fluorescencemonitor wavelength of 440 nm and 520 nm. The results are illustrated inFIG. 11.

In FIG. 11, the result as to the excitation wavelength of 350 nm isindicated by the fluorescence spectrum (i), the result as to thefluorescence monitor wavelength of 440 nm is indicated by the spectrum(ii), and the result as to the fluorescence monitor wavelength of 520 nmis indicated by the spectrum (iii).

In FIG. 11, fluorescence specific to the copper clusters are observed,and it is therefore understood that the copper clusters are supported onthe carrier particle.

Example 13 and Comparative Example 5

In Example 13 and Comparative Example 5, rhodium cluster-supportingcatalysts were obtained by using a method of laser ablation in liquidand an ion exchange-reduction method, respectively.

Example 13

In Example 13, rhodium clusters were supported on a zeolite carrierparticle by a method of laser ablation in liquid in the same manner asin Example 1 except that a rhodium target was used in place of the goldtarget and ZSM-5 zeolite carrier particles (Si/Al ratio: 40) were usedas the zeolite carrier particle.

Comparative Example 5

In Comparative Example 5, a rhodium ion was supported on ZSM-5 zeolitecarrier particles (Si/Al ratio: 40) by ion exchange, and then a metallicrhodium particle was supported on the zeolite carrier particle byreducing the rhodium ion, whereby rhodium clusters were supported on thezeolite carrier particle (ion exchange-reduction method). Rh(NO₃)₃ wasused as a rhodium ion source and NaBH₄ was used as a reducing agent.

<Evaluation: Fluorescence Spectrum>

The supported catalysts of Example 13 and Comparative Example 5 weremeasured for the fluorescence spectrum (excitation wavelength: 350 nm).The evaluation results of fluorescence spectrum normalized to theintensity per 1 mg of rhodium are illustrated in FIG. 12.

It is understood from FIG. 12 that compared with Comparative Example 5using an ion exchange-reduction method, in Example 13 using laserablation in liquid, the fluorescence peak is large, i.e., a relativelylarge number of rhodium particles are supported in the cluster state onthe zeolite carrier particle.

Example 14 and Comparative Example 6

In Example 14 and Comparative Example 6, supported gold-clustercatalysts were obtained by using a method of laser ablation in liquidand an ion exchange-reduction method, respectively.

Example 14

In Example 14, gold clusters were supported on a zeolite carrierparticle by a method of laser ablation in liquid in the same manner asin Example 1 except that ZSM-5 zeolite carrier particles (MFI) (Si/Alratio: 1,500) were used as the zeolite carrier particle.

Comparative Example 6

In Comparative Example 6, a gold ion was supported on ZSM-5 zeolitecarrier particles (MFI) (Si/Al ratio: 1,500) by ion exchange, and thengold clusters were supported on the zeolite carrier particle by reducingthe gold ion (ion exchange-reduction method). Chloroauric acid (HAuCl₄)was used as a gold ion source and NaBH₄ was used as a reducing agent.

<Evaluation: Overall Composition Evaluation (ICP-OES)>

With respect to the supported catalysts of Example 14 and ComparativeExample 6, the elemental composition of the supported catalyst as awhole was evaluated using inductively coupled plasma spectrometer(ICP-OES apparatus) (Agilent 5100 manufactured by Agilent Technologies,Inc. and SPS3000 manufactured by Hitachi High-Tech Science Corporation).The results are shown in Table 2 below.

<Evaluation: Surface Composition Evaluation (TEM-EDX)>

With respect to the supported catalysts of Example 14 and ComparativeExample 6, the elemental composition of the supported catalyst surfacewas evaluated using a transmission electron microscope-energy dispersiveX-ray spectroscopy (TEM-EDX) (JEM-2100F and JED-2300, manufactured byJEOL Ltd.). The results are shown in Table 2 below.

TABLE 2 Comparative Example 14 Example 6 (Au laser (Au ion change-ablation/ reduction/ MFI(1500)) MFI(1500)) Whole Au (wt %) 0.015 0.005(ICP-MASS) Si (wt %) 38 41 Au/Si 3.95 × 10⁻⁴ 1.22 × 10⁻⁴ Surface Au (wt%) 0.46 0.44 (TEM-EDX) Si (wt %) 66.94 59.82 Au/Si 6.87 × 10⁻³ 7.36 ×10⁻³ Heterogeneity (Au/Si (surface))/ 17.39 60.33 index (Au/Si (whole))

It is understood from Table 2 that in the supported catalyst of Example14 obtained by the method of laser ablation in liquid, compared with thesupported catalyst of Comparative Example 6 obtained by the ionexchange-reduction method, the ratio of the proportion of gold elementin the whole to the proportion of gold element in the surface is small,i.e., the gold clusters are relatively uniformly dispersed in thesupported catalyst.

Example 15 and Comparative Example 7

In Example 15 and Comparative Example 7, rhodium cluster-supportingcatalysts were obtained by using a method of laser ablation in liquidand an ion exchange-reduction method, respectively.

Example 15

In Example 15, rhodium clusters were supported on a zeolite carrierparticle by a method of laser ablation in liquid in the same manner asin Example 1 except that a rhodium target was used in place of the goldtarget and ZSM-5 zeolite carrier particles (Si/Al ratio: 40) were usedas the zeolite carrier particle. The amount of rhodium supported was 0.1mass % relative to the zeolite carrier particle.

Comparative Example 7

In Comparative Example 7, a rhodium ion was supported on ZSM-5 zeolitecarrier particles (Si/Al ratio: 40) by ion exchange, and then a metallicrhodium particle was supported on the zeolite carrier particle byreducing the rhodium ion, whereby rhodium clusters were supported on thezeolite carrier particle (ion exchange-reduction method). Rh(NO₃)₃ wasused as a rhodium ion source and NaBH₄ was used as a reducing agent. Theamount of rhodium supported was 0.051 mass % relative to the zeolitecarrier particle.

<Evaluation: H₂-TPR Test (Before Thermal Endurance)>

With respect to the supported catalysts of Example 15 and ComparativeExample 7, a pretreatment was performed by adsorbing oxygen to thesupported catalyst at 30° C. for 1 hour in a 100 vol % oxygen atmosphereand removing excess oxygen at 500° C. for 1 hour in a helium atmosphere.

With respect to the supported catalysts above subjected to thepretreatment, a test by hydrogen temperature-programmed reduction method(H₂-TPR) was performed by flowing a reducing gas containing 0.5 vol % ofhydrogen and the balance helium at a spatial velocity of 10,000 h⁻¹while raising the temperature at a rate of 10° C./min from 20° C.

The result as to the supported catalyst of Example 15 is illustrated inFIG. 13A, and the result as to the supported catalyst of ComparativeExample 7 is illustrated in FIG. 14A. The peak/noise ratio of the peakindicated by an arrow of FIG. 13A was 35.7 (noise level: 0.000215%), andthe peak/noise ratio of the peak indicated by an arrow of FIG. 13B was5.12 (noise level: 0.000394%).

It is understood from these graphs that both of the supported catalystsof Example 15 and Comparative Example 7 have a relatively large peak ofreaction between hydrogen supplied and oxygen adsorbed to thecluster-supporting catalyst, i.e., a peak with a peak/noise ratio of 2.0or more, in the temperature range of 150° C. or less, i.e., havelow-temperature activity.

<Evaluation: H₂-TPR Test (after thermal endurance)>

With respect to the supported catalysts of Example 15 and ComparativeExample 7, a thermal endurance treatment was performed by subjecting thesupported catalyst to heating for 2 hours in an atmosphere at 800° C.containing 20 vol % of oxygen and the balance helium and then to heatingfor 1 hour in an atmosphere at 800° C. containing 0.5 vol % of hydrogenand the balance helium.

With respect to the supported catalysts above subjected to the thermalendurance treatment, a pretreatment was performed as described above.

With respect to the supported catalysts above subjected to thepretreatment, a H₂-TPR test was performed as described above.

The result as to the supported catalyst of Example 15 is illustrated inFIG. 13B, and the result as to the supported catalyst of ComparativeExample 7 is illustrated in FIG. 14B. The peak/noise ratio of the peakindicated by an arrow of FIG. 14A was 7.76 (noise level: 0.000326%), andthe peak/noise ratio of the peak indicated by an arrow of FIG. 14B was1.62 (noise level: 0.000377%).

It is understood from FIG. 13B that the supported catalyst of Example 15has a relatively large reaction peak in the temperature range of 150° C.or less, i.e., has low-temperature activity. In addition, it isunderstood from FIG. 14B that the supported catalyst of ComparativeExample 7 does not have a substantial peak in the temperature range of150° C. or less, i.e., does not have a peak with a peak/noise ratio of2.0 or more. In this way, the supported catalyst of Comparative Example7 does not have a substantial peak in the temperature range of 150° C.or less, and this means that the supported catalyst does not havelow-temperature activity. That is, it is understood that in thesupported catalyst of Comparative Example 7 obtained by the ionexchange-reduction method, the dispersibility of the cluster particlewas low and in turn, the heat resistance was poor.

Example 16 and Comparative Example 8

In Example 16 and Comparative Example 8, palladium cluster-supportingcatalysts were obtained by using a method of laser ablation in liquidand an ion exchange-reduction method, respectively.

Example 16

In Example 16, palladium clusters were supported on a zeolite carrierparticle by a method of laser ablation in liquid in the same manner asin Example 1 except that a palladium target was used in place of thegold target and ZSM-5 zeolite carrier particles (Si/Al ratio: 40) wereused as the zeolite carrier particle. The amount of palladium supportedwas 0.09 mass % relative to the zeolite carrier particle.

Comparative Example 8

In Comparative Example 8, a palladium ion was supported on ZSM-5 zeolitecarrier particles (Si/Al ratio: 40) by ion exchange, and then a metallicpalladium particle was supported on the zeolite carrier particle byreducing the palladium ion, whereby palladium clusters were supported onthe zeolite carrier particle (ion exchange-reduction method).Pd(NH₃)₄Cl₂.H₂O (tetraamminepalladium(II) chloride monohydrate) was usedas a palladium ion source and NaBH₄ was used as a reducing agent. Theamount of palladium supported was 0.86 mass % relative to the zeolitecarrier particle.

<Evaluation: Carbon Monoxide Oxidation Test>

With respect to the supported catalysts of Example 16 and ComparativeExample 8, a thermal endurance treatment was performed by subjecting thesupported catalyst to heating for 2 hours in an atmosphere at 800° C.containing 20 vol % of oxygen and the balance helium and then to heatingfor 1 hour in an atmosphere at 800° C. containing 0.5 vol % of hydrogenand the balance helium.

A model gas containing 0.3 vol % of carbon monoxide, 8.0 vol % of oxygenand the balance helium was flowed at a spatial velocity of 10,000 h⁻¹over the supported catalysts subjected to the thermal endurancetreatment, and the number of molecules of the carbon monoxide moleculecapable of being oxidized to carbon dioxide molecule by one palladiumatom at a temperature of 100° C. in the temperature dropping process wasevaluated by performing a temperature rising process of raising thetemperature at a rate of 10° C./min to 800° C. from room temperature andthen performing a temperature dropping process of lowering thetemperature to room temperature.

The number of molecules can be obtained by dividing the molar number ofthe carbon dioxide molecule in the model gas flowing per second afterreaction by the molar number of palladium as a catalyst metal in thesupported catalyst.

The results as to the supported catalysts of Example 16 and ComparativeExample 8 are illustrated in FIG. 15. It is seen from FIG. 15 that inthe supported catalyst of Example 16 obtained by the laser ablationmethod, the number of molecules of the carbon monoxide molecule capableof being oxidized to carbon dioxide molecule by one palladium atom wasnear 0.008 and on the other hand, in the supported catalyst ofComparative Example 8 obtained by the ion exchange-reduction method, thenumber did not reach 0.002. This indicates that in the supportedcatalyst of Comparative Example 8 obtained by the ion exchange-reductionmethod, the dispersibility of the cluster particle was low and in turn,the heat resistance was poor.

Example 17 and Comparative Example 9

In Example 17 and Comparative Example 9, platinum cluster-supportingcatalysts were obtained by using a method of laser ablation in liquidand an ion exchange-reduction method, respectively.

Example 17

In Example 17, platinum clusters were supported on a zeolite carrierparticle by a method of laser ablation in liquid in the same manner asin Example 1 except that a platinum target was used in place of the goldtarget and ZSM-5 zeolite carrier particles (Si/Al ratio: 40) were usedas the zeolite carrier particle. The amount of platinum supported was1.1 mass % relative to the zeolite carrier particle.

Comparative Example 9

In Comparative Example 9, a platinum ion was supported on ZSM-5 zeolitecarrier particles (Si/Al ratio: 40) by ion exchange, and then a metallicplatinum particle was supported on the zeolite carrier particle byreducing the platinum ion, whereby platinum clusters were supported onthe zeolite carrier particle (ion exchange-reduction method).Pt(NH₃)₄Cl₂.xH₂O (tetraammineplatinum(II) chloride monohydrate) was usedas a platinum ion source and NaBH₄ was used as a reducing agent. Theamount of platinum supported was 1.9 mass % relative to the zeolitecarrier particle.

<Evaluation: Carbon Monoxide Oxidation Test>

With respect to the supported catalysts of Example 17 and ComparativeExample 9, a thermal endurance treatment was performed by subjecting thesupported catalyst to heating for 2 hours in an atmosphere at 800° C.containing 20 vol % of oxygen and the balance helium and then to heatingfor 1 hour in an atmosphere at 800° C. containing 0.5 vol % of hydrogenand the balance helium.

A model gas containing 0.3 vol % of carbon monoxide, 8.0 vol % of oxygenand the balance helium was flowed at a spatial velocity of 10,000 h⁻¹over the supported catalysts subjected to the thermal endurancetreatment, and the number of molecules of the carbon monoxide moleculecapable of being oxidized to carbon dioxide molecule by one platinumatom at a temperature of 60° C. in the temperature dropping process wasevaluated by performing a temperature rising process of raising thetemperature at a rate of 10° C./min to 800° C. from room temperature andthen performing a temperature dropping process of lowering thetemperature to room temperature.

The results as to the supported catalysts of Example 17 and ComparativeExample 9 are illustrated in FIG. 16. It is seen from FIG. 16 that inthe supported catalyst of Example 17 obtained by the laser ablationmethod, the number of molecules of the carbon monoxide molecule capableof being oxidized to carbon dioxide molecule by one platinum atom wasnear 0.0002 and on the other hand, in the supported catalyst ofComparative Example 9 obtained by the ion exchange-reduction method, thenumber did not reach 0.0001. This indicates that in the supportedcatalyst of Comparative Example 9 obtained by the ion exchange-reductionmethod, the dispersibility of the cluster particle was low and in turn,the heat resistance was poor.

Example 18 and Comparative Example 10

In Example 18 and Comparative Example 10, supported copper-clustercatalysts were obtained by using a method of laser ablation in liquidand an ion exchange-reduction method, respectively.

Example 18

In Example 18, copper clusters were supported on a zeolite carrierparticle by a method of laser ablation in liquid in the same manner asin Example 1 except that a copper target was used in place of the goldtarget and a chavazite (CHA)-type zeolite carrier particle were used asthe zeolite carrier particle. The amount of copper supported was 0.9mass % relative to the zeolite carrier particle.

Comparative Example 10

In Comparative Example 10, a copper ion was supported on a chavazite(CHA)-type zeolite carrier particle by ion exchange, and then a metalliccopper particle was supported on the zeolite carrier particle byreducing the copper ion, whereby copper clusters were supported on thezeolite carrier particle (ion exchange-reduction method). Copper nitratewas used as a copper ion source and NaBH₄ was used as a reducing agent.The amount of copper supported was 0.9 mass % relative to the zeolitecarrier particle.

<Evaluation: Nitric Oxide Temperature-Programmed Desorption Test>

With respect to the supported catalysts of Example 18 and ComparativeExample 10, nitric oxide was adsorbed to the supported catalyst throughheating at 800° C. for 1 hour in an atmosphere containing 10 vol % ofoxygen and the balance helium, heating at 800° C. for 30 minutes in anatmosphere containing 100 vol % of helium, lowering of the ambienttemperature to 25° C., holding for 1 hour in an atmosphere containing500 ppm by volume of nitric oxide and the balance helium, and holdingfor 1 hour in an atmosphere containing 100 vol % of helium.

The supported catalyst having adsorbed thereto nitric oxide was heatedat a temperature rise rate of 10° C./min to 800° C. in an atmospherecontaining 100 vol % of helium, and the amount of nitric oxide desorbedduring the heating was detected by a mass spectrometer to obtain anitric oxide temperature-programmed desorption spectrum. Incidentally,the gas flow rate in the atmosphere was 10 sccm in all cases.

The results as to the supported catalysts of Example 18 and ComparativeExample 10 are illustrated in FIGS. 17A and 17B, respectively.

It is revealed from FIG. 17A that in the supported catalyst of Example18 obtained by the laser ablation method, the maximum peak in the rangeof 200 to 400° C. is present at about 270° C. On the other hand, it isrevealed from FIG. 17B that in the supported catalyst of ComparativeExample 10 obtained by the ion exchange-reduction method, the maximumpeak in the range of 200 to 400° C. is present at about 320° C. A sharppeak observed at a temperature of about 200° C. or less is thought of asa measurement error due to fluctuation of the measurement temperature.

The difference in the temperature between maximum peaks illustrated inFIGS. 17A and 17B indicates that the supported catalyst of Example 18obtained by the laser ablation method and the supported catalyst ofComparative Example 10 obtained by the ion exchange-reduction methodhave different structures from each other.

Example 19 and Comparative Example 11

In Example 19 and Comparative Example 11, platinum cluster-supportingcatalysts were obtained by using an positive-negative inversion methodand an ion exchange-reduction method, respectively.

Example 19

In Example 19, zeolite MFI(40) was added to 200 ml of an aqueoussolution containing 10 mM of H₂[PtCl₆] in pure water, a pulsed laser isconverged and introduced into this aqueous solution to decomposeH₂[PtCl₆] and produce a positively charged platinum cluster, and thepositively charged platinum clusters were supported on the acid sites ofzeolite through an electrostatic interaction.

Comparative Example 11

In Comparative Example 11, H₂[PtCl₆] in pure water was supported onzeolite MFI(40) by ion exchange. The amount of platinum supported was0.003 mass % relative to the zeolite carrier particle.

<Evaluation: Fluorescence Spectrum>

The platinum cluster-supporting catalysts of Example 19 and ComparativeExample 11 were measured for the fluorescence spectrum (excitationwavelength: 350 nm). The evaluation results of fluorescence spectrum areillustrated in FIG. 18. In FIG. 18, the results as to Example 18 isindicated by the spectrum (i), and the result as to Comparative Example11 is indicated by the spectrum (ii).

In FIG. 18, the fluorescence signal at near 410 nm is a spectrum inwhich fluorescent emissions from the gold cluster of about tetramer wereoverlapped. Accordingly, FIG. 18 reveals that in the platinumcluster-supporting catalyst of Example 18, a relatively large amount ofa platinum cluster around tetramer is supported on the carrier particleand on the other hand, in the supported-platinum catalyst of ComparativeExample 11, such clusters are not present in a significant manner.

Example 20 and Comparative Example 12

In Example 20 and Comparative Example 12, rhodium cluster-supportingcatalysts were obtained by using a method of laser ablation in liquidand an ion exchange-reduction method, respectively.

Example 20

In Example 20, rhodium clusters were supported on a zeolite carrierparticle by a method of laser ablation in liquid in the same manner asin Example 1 except that a rhodium target was used in place of the goldtarget and ZSM-5 zeolite carrier particles (Si/Al ratio: 40) were usedas the zeolite carrier particle. The amount of rhodium supported was 0.1mass % relative to the zeolite carrier particle.

Comparative Example 12

In Comparative Example 12, a rhodium ion was supported on ZSM-5 zeolitecarrier particles (Si/Al ratio: 40) by ion exchange, and then rhodiumclusters were supported on the zeolite carrier particle by reducing therhodium ion. Rh(NO₃)₃ was used as a rhodium ion source and NaBH₄ wasused as a reducing agent. The amount of rhodium supported was 0.051 mass% relative to the zeolite carrier particle.

<Evaluation: Nitric Oxide Reduction Test>

A thermal endurance treatment was performed by heating the supportedcatalyst for 1 hour in an atmosphere at 800° C. containing 8 vol % ofoxygen, 0.3 vol % of carbon monoxide and the balance helium.

A model gas containing 0.1 vol % of ¹⁵NO, 0.65 vol % of CO and thebalance helium was flowed at a spatial velocity of 10,000 h⁻¹ over thesupported catalysts subjected to the thermal endurance treatment, andthe nitric oxide reduction reaction was measured by raising thetemperature at a rate of 10° C./min to 800° C. from room temperature(temperature rising process) and then lowering the temperature to roomtemperature (temperature dropping process).

With respect to the concentration change of each component due to thenitric oxide reduction, the evaluation result as to the catalyst ofExample 20 is illustrated in FIG. 19A, and the evaluation result as tothe catalyst of Comparative Example 12 is illustrated in FIG. 19B.

In FIGS. 19A and 19B, a peak of nitrogen oxide appears in the range of100 to 200° C., and this is a concentration rise due to desorption ofnitric oxide adsorbed to the catalyst. When the reaction temperaturefurther rises, the concentration of nitric oxide decreases, and areaction of reducing nitric oxide (¹⁵NO) by carbon monoxide (CO) toproduce nitrogen (N₂) starts.

In the catalyst of Example 20, the reaction temperature at the time ofhalf of the nitric oxide supplied being reduced into nitrogen, i.e., thereaction temperature at the time of the nitrogen concentration becoming0.05 vol %, is about 272° C. in the temperature rising process and 254°C. in the temperature dropping process, whereas in the catalyst ofComparative Example 12, the reaction temperature was about 321° C. inthe temperature rising process and 279° C. in the temperature droppingprocess. It is therefore revealed that the catalyst of Example 20obtained by the method of laser ablation in liquid has excellentlow-temperature activity, compared with the catalyst of ComparativeExample 12 obtained by the ion exchange-reduction method.

The evaluation results of the number of molecules of the nitric oxidemolecule capable of being reduced to nitrogen by one rhodium atom at atemperature of 250° C. in the temperature dropping process areillustrated in FIG. 20.

FIG. 20 reveals that in the supported catalyst of Example 20 obtained bythe laser ablation method, the number of molecules of nitrogen monoxidemolecule capable of being purified in 1 second by one rhodium atomexceeded 0.007 and on the other hand, in the catalyst of ComparativeExample 12 obtained by the ion exchange-reduction method, the number didnot reach 0.004. It is therefore apparent that the catalyst of Example20 obtained by the laser ablation method has excellent low-temperatureactivity, compared with the catalyst of Comparative Example 12 obtainedby the ion exchange-reduction method.

Examples 21 and 22 and Comparative Example 13

In Examples 21 and 22, a platinum cluster-supporting catalyst and arhodium cluster-supporting catalyst were obtained respectively by usinga method of laser ablation in liquid. In Comparative Example 13, ageneral three-way catalyst in which platinum, rhodium and palladium aresupported on a mixed powder of alumina carrier particles andceria-zirconia carrier particles was used.

Example 21

In Example 21, platinum clusters were supported on a zeolite carrierparticle by a method of laser ablation in liquid in the same manner asin Example 1 except that a platinum target was used in place of the goldtarget and ZSM-5 zeolite carrier particles (Si/Al ratio: 40) were usedas the zeolite carrier particle. The amount of platinum supported was0.59 mass % relative to the zeolite carrier particle.

Example 22

In Example 22, rhodium clusters were supported on a zeolite carrierparticle by a method of laser ablation in liquid in the same manner asin Example 1 except that a rhodium target was used in place of the goldtarget and ZSM-5 zeolite carrier particles (Si/Al ratio: 40) were usedas the zeolite carrier particle. The amount of rhodium supported was 0.1mass % relative to the zeolite carrier particle.

Comparative Example 13

In Comparative Example 13, a general three-way catalyst in whichplatinum, rhodium and palladium are supported on a mixed powder ofalumina carrier particles and ceria-zirconia carrier particles was used.The amounts of platinum, rhodium and palladium supported were 0.2 mass%, 0.19 mass % and 0.25 mass %, respectively, relative to the carrierpowder.

<Evaluation: Oxygen Oxidation Reaction Test of Adsorbed Carbon Monoxide>

The catalysts of Examples 21 and 22 and Comparative Example 13 were heldat 800° C. for 1 hour in an atmosphere containing 500 ppm by volume ofcarbon monoxide and the balance helium to adsorb carbon monoxide to thesupported catalyst, and then an oxygen oxidation reaction test ofadsorbed carbon monoxide was performed by heating the supported catalysthaving adsorbed thereto carbon monoxide at a temperature rise rate of10° C./min to 800° C. in an atmosphere containing 10 vol % of oxygen andthe balance helium. During these treatments, the spatial velocity was10,000 h⁻¹.

In addition, the catalysts of Examples 21 and 22 and Comparative Example13 were cleaned by performing the following treatments (i) to (iv):

(i) putting the catalyst at a concentration of 4 mass % in an aqueous 1M sodium chloride solution, followed by stirring at 80° C. for 10 days,

(ii) after (i) above, rinsing the catalyst with ion-exchanged water,

(iii) after (ii) above, putting the catalyst at a concentration of 4mass % in an aqueous solution containing 6 mass % of polyoxyethylenesorbitan monolaurate, 0.25 M trisodium ethylenediaminetetraacetate, and0.01 M sodium borohydride, followed by stirring at 80° C. for 10 days,and

(iv) after (iii) above, rinsing the catalyst with ion-exchanged water.

The catalysts of Examples 21 and 22 and Comparative Example 13 subjectedto the cleaning treatment were subjected to the above-described oxygenoxidation reaction test of adsorbed carbon monoxide.

With respect to the catalysts of Examples 21 and 22 and ComparativeExample 13, the results of the oxygen oxidation reaction test ofadsorbed carbon monoxide before and after the cleaning treatment areillustrated in FIG. 21.

As apparent from FIG. 21, in the catalysts of Examples 21 and 22obtained by the method of laser ablation in liquid, a signal on the lowtemperature side of 200° C. or less was present before and after thecleaning treatment, whereas in the catalyst of Comparative Example 13that is a general three-way catalyst, the difference in the evaluationresult between before and after the cleaning treatment was large and asignal on the low temperature side of 200° C. or less was not observed.

This is considered to be attributable to the fact that in the catalystsof Examples 21 and 22 obtained by the method of laser ablation inliquid, the catalyst metal clusters was supported within the pores ofzeolite and the catalyst metal clusters was thereby not lost even bycleaning, whereas in the general three-way catalyst, the catalyst metalparticles were supported on the outer surface of the carrier particleand consequently, the catalyst metal particles were lost by cleaning.

DESCRIPTION OF NUMERICAL REFERENCES

-   11 Acetone as dispersion medium-   12 Plate of gold-   13 Vessel 13-   14 Lens 14-   15 Laser-   16 Gold cluster-   20 Zeolite carrier particle

1. A cluster-supporting catalyst comprising porous carrier particleshaving acid sites, and catalyst metal clusters supported within thepores of the porous carrier particles, and satisfying any of thefollowing (a) to (d): (a) the catalyst metal is rhodium and satisfies atleast one of the following (a1) to (a3): (a1) when thecluster-supporting catalyst is subjected to a first thermal endurancetreatment for rhodium, then to an oxygen adsorption pretreatment andfurther to a test by hydrogen temperature-programmed reduction method,the peak of reaction between hydrogen supplied and oxygen adsorbed tothe cluster-supporting catalyst is present in the temperature range of150° C. or less, wherein the first thermal endurance treatment forrhodium is a treatment of subjecting the cluster-supporting catalyst toheating for 2 hours in an atmosphere at 800° C. containing 20 vol % ofoxygen and the balance helium and then to heating for 1 hour in anatmosphere at 800° C. containing 0.5 vol % of hydrogen and the balancehelium, the oxygen adsorption pretreatment is a treatment of adsorbingoxygen to the cluster-supporting catalyst at 30° C. for 1 hour in anoxygen atmosphere and removing excess oxygen at 500° C. for 1 hour in ahelium atmosphere, and the test by hydrogen temperature-programmedreduction method is a test of flowing a reducing gas containing 0.5 vol% of hydrogen and the balance helium at a spatial velocity of 10,000 h⁻¹over the cluster-supporting catalyst while raising the temperature at arate of 10° C./min from 20° C.; (a2) when the cluster-supportingcatalyst is subjected to a second thermal endurance treatment forrhodium and then to a nitric oxide reduction test, (i) the reactiontemperature at the time of half of the nitric oxide supplied beingreduced into nitrogen satisfies at least either one of 300° C. or lessin the temperature rising process and 270° C. or less in the temperaturedropping process, and/or (ii) the number of molecules of the nitricoxide molecule when nitric oxide can be reduced by one rhodium atom at atemperature of 250° C. in the temperature dropping process is 0.005molecules/sec or more, wherein the second thermal endurance treatmentfor rhodium is a treatment of heating the cluster-supporting catalystfor 1 hour in an atmosphere at 800° C. containing 8 vol % of oxygen, 0.3vol % of carbon monoxide and the balance helium, and the nitric oxidereduction test is a test of flowing a model gas containing 0.1 vol % of¹⁵NO, 0.65 vol % of CO and the balance helium at a spatial velocity of10,000 h⁻¹, performing a temperature rising process of raising thetemperature at a rate of 10° C./min to 800° C. from room temperature,and then performing a temperature dropping process of lowering thetemperature to room temperature; and (a3) when the cluster-supportingcatalyst is subjected to a cleaning treatment and then to an adsorbedcarbon monoxide oxidation test, the peak of reaction between carbonmonoxide adsorbed to the cluster-supporting catalyst and oxygen in theatmosphere is present in the temperature range of 200° C. or less,wherein the cleaning treatment consists of the following steps (i) to(iv): (i) putting the catalyst at a concentration of 4 mass % in anaqueous 1 M sodium chloride solution, followed by stirring at 80° C. for10 days, (ii) after (i) above, rinsing the catalyst with ion-exchangedwater, (iii) after (ii) above, putting the catalyst at a concentrationof 4 mass % in an aqueous solution containing 6 mass % ofpolyoxyethylene sorbitan monolaurate, 0.25 M trisodiumethylenediaminetetraacetate, and 0.01 M sodium borohydride, followed bystirring at 80° C. for 10 days, and (iv) after (iii) above, rinsing thecatalyst with ion-exchanged water, and the adsorbed carbon monoxideoxidation test is a test of adsorbing carbon monoxide to thecluster-supporting catalyst by holding the cluster-supporting catalystat 800° C. for 1 hour in an atmosphere containing 500 ppm by volume ofcarbon monoxide and the balance helium, and thereafter oxidizing thecarbon monoxide adsorbed to the cluster-supporting catalyst into carbondioxide with oxygen by heating the cluster-supporting catalyst havingadsorbed thereto carbon monoxide at a rate of 10° C./min to 800° C. inan atmosphere containing 10 vol % of oxygen and the balance helium; (b)the catalyst metal is palladium and when the cluster-supporting catalystis subjected to a thermal endurance treatment for palladium and then toa carbon monoxide purification test for palladium, the number ofmolecules of the carbon monoxide molecule capable of being oxidized tocarbon dioxide molecule by one palladium atom is 0.004 molecules/sec ormore, wherein the thermal endurance treatment for palladium is atreatment of heating the cluster-supporting catalyst for 10 hours in anatmosphere at 800° C. containing 20 vol % of oxygen and the balancehelium, and the carbon monoxide purification test for palladium is atest of flowing a model gas containing 0.3 vol % of carbon monoxide, 8.0vol % of oxygen and the balance helium at a spatial velocity of 10,000h⁻¹ over the cluster-carried catalyst, performing a temperature risingprocess of raising the temperature at a rate of 10° C./min to 800° C.from room temperature, then performing a temperature dropping process oflowering the temperature to room temperature, and measuring thecatalytic activity at a temperature of 100° C. in the temperaturedropping process; (c) the catalyst metal is platinum and satisfies atleast one of the following (c1) and (c2): (c1) when thecluster-supporting catalyst is subjected to a thermal endurancetreatment for platinum and then to a carbon monoxide purification testfor platinum, the number of molecules of the carbon monoxide moleculecapable of being oxidized to carbon dioxide molecule by one platinumatom is 0.00015 molecules/sec or more, wherein the thermal endurancetreatment for platinum is a treatment of heating the cluster-supportingcatalyst for 10 hours in an atmosphere at 800° C. containing 20 vol % ofoxygen and the balance helium, and the carbon monoxide purification testfor platinum is a test of flowing a model gas containing 0.3 vol % ofcarbon monoxide, 8.0 vol % of oxygen and the balance helium at a spatialvelocity of 10,000 h⁻¹ over the cluster-carried catalyst, performing atemperature rising process of raising the temperature at a rate of 10°C./min to 800° C. from room temperature, then performing a temperaturedropping process of lowering the temperature to room temperature, andmeasuring the catalytic activity at a temperature of 60° C. in thetemperature dropping process, and (c2) when the cluster-supportingcatalyst is subjected to a cleaning treatment and then to an adsorbedcarbon monoxide oxidation test, the peak of reaction between carbonmonoxide adsorbed to the cluster-supporting catalyst and oxygen in theatmosphere is present in the temperature range of 200° C. or less,wherein the cleaning treatment consists of the following steps (i) to(iv): (i) putting the catalyst at a concentration of 4 mass % in anaqueous 1 M sodium chloride solution, followed by stirring at 80° C. for10 days, (ii) after (i) above, rinsing the catalyst with ion-exchangedwater, (iii) after (ii) above, putting the catalyst at a concentrationof 4 mass % in an aqueous solution containing 6 mass % ofpolyoxyethylene sorbitan monolaurate, 0.25 M trisodiumethylenediaminetetraacetate, and 0.01 M sodium borohydride, followed bystirring at 80° C. for 10 days, and (iv) after (iii) above, rinsing thecatalyst with ion-exchanged water, and the adsorbed carbon monoxideoxidation test is a test of adsorbing carbon monoxide to thecluster-supporting catalyst by holding the cluster-supporting catalystat 800° C. for 1 hour in an atmosphere containing 500 ppm by volume ofcarbon monoxide and the balance helium, and thereafter oxidizing thecarbon monoxide adsorbed to the cluster-supporting catalyst into carbondioxide with oxygen by heating the cluster-supporting catalyst havingadsorbed thereto carbon monoxide at a rate of 10° C./min to 800° C. inan atmosphere containing 10 vol % of oxygen and the balance helium; and(d) the catalyst metal is copper and when the cluster-supportingcatalyst is subjected to a nitric oxide temperature-programmeddesorption test, the maximum peak in the range of 200 to 400° C. ispresent in the range of 200 to 300° C., wherein the nitric oxidetemperature-programmed desorption test is a test of adsorbing nitricoxide to the supported catalyst through heating at 800° C. for 1 hour inan atmosphere containing 10 vol % of oxygen and the balance helium,heating at 800° C. for 30 minutes in an atmosphere containing 100 vol %of helium, lowering of the ambient temperature to 25° C., holding for 1hour in an atmosphere containing 500 ppm by volume of nitric oxide andthe balance helium, and holding for 1 hour in an atmosphere containing100 vol % of helium, and thereafter heating the supported catalysthaving adsorbed thereto nitric oxide at a temperature rise rate of 10°C./min to 800° C. in an atmosphere containing 100 vol % of helium. 2.The cluster-supporting catalyst according to claim 1, which satisfies(a) above.
 3. The cluster-supporting catalyst according to claim 1,which satisfies (b) above.
 4. The cluster-supporting catalyst accordingto claim 1, which satisfies (c) above.
 5. The cluster-supportingcatalyst according to claim 1, which satisfies (d) above.